[PATCH 00/12] [PATCH v14 00/12] migrate on fault for device pages

[PATCH 00/12] [PATCH v14 00/12] migrate on fault for device pages

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

Currently, the way device page faulting and migration works
is not optimal, if you want to do both fault handling and
migration at once.

Being able to migrate not present pages (or pages mapped with incorrect
permissions, eg. COW) to the GPU requires doing either of the
following sequences:

1. hmm_range_fault() - fault in non-present pages with correct permissions, etc.
2. migrate_vma_*() - migrate the pages

Or:

1. migrate_vma_*() - migrate present pages
2. If non-present pages detected by migrate_vma_*():
   a) call hmm_range_fault() to fault pages in
   b) call migrate_vma_*() again to migrate now present pages

The problem with the first sequence is that you always have to do two
page walks even when most of the time the pages are present or zero page
mappings so the common case takes a performance hit.

The second sequence is better for the common case, but far worse if
pages aren't present because now you have to walk the page tables three
times (once to find the page is not present, once so hmm_range_fault()
can find a non-present page to fault in and once again to setup the
migration). It is also tricky to code correctly. One page table walk
could costs over 1000 cpu cycles on X86-64, which is a significant hit.

We should be able to walk the page table once, faulting
pages in as required and replacing them with migration entries if
requested.

Add a new flag to HMM APIs, HMM_PFN_REQ_MIGRATE,
which tells to prepare for migration also during fault handling.
For the migrate_vma_setup() call paths, new flags, MIGRATE_VMA_FAULT,
and MIGRATE_VMA_WRITE are added to tell to add fault handling to migrate.

An extra benefit of migrating with hmm_range_fault() path
is the migrate_vma.vma gets populated, so no need to
retrieve that separataly.

Tested in X86-64 VM with HMM test device, passing the selftests.
For performance, the migrate throughput tests from the selftests
show similar numbers (within error margin) as unmodified kernel.
Tested also rebased on the
"Remove device private pages from physical address space" series:
https://lore.kernel.org/linux-mm/20260130111050.53670-1-jniethe@nvidia.com/
plus a small patch to adjust with no problems.

Changes since v13:
  - rebased on v7.3-rc
  - fix compile error in !CONFIG_MMU_NOTIFIER kernel configs
  - added patch 12/12 for Documentation/mm/hmm updates
  - document the semantics of new flags for migrate_vma_setup()
  - fix error handling after migrate_vma_split_folio() failure
  - comment and style fixes

Link to v13: https://lore.kernel.org/linux-mm/20260804042631.2175585-1-mpenttil@redhat.com/

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>

Mika Penttilä (12):
  mm/Kconfig: changes for migrate on fault for device pages
  mm: add helper to convert HMM pfn to migrate pfn
  mm/hmm: preparations for HMM to participate in migration
  mm/hmm: do the plumbing for HMM to participate in migration
  mm/hmm: implement folio split for migrate needs in HMM pagewalk
  mm/hmm: migrate collection in HMM pagewalk - pte level
  mm/hmm: migrate collection in HMM pagewalk - pmd level
  mm/hmm: add lazy MMU mode support for migration in HMM pagewalk
  mm/hmm: implement rollback for device page migration in HMM pagewalk
  mm: enable device page migration from HMM pagewalk
  lib/test_hmm: add a new testcase for the migrate on fault
  Documentation/mm/hmm: document migration through hmm_range_fault()

 Documentation/mm/hmm.rst               |  39 +
 include/linux/hmm.h                    |  51 +-
 include/linux/migrate.h                |  58 +-
 lib/test_hmm.c                         | 132 +++-
 lib/test_hmm_uapi.h                    |  21 +-
 mm/Kconfig                             |   1 +
 mm/hmm.c                               | 977 +++++++++++++++++++++++--
 mm/migrate_device.c                    | 617 +++-------------
 tools/testing/selftests/mm/hmm-tests.c |  54 ++
 9 files changed, 1341 insertions(+), 609 deletions(-)

drm-tip
base-commit: dfe5a8188de9aaddd4e46b0f2410d0bccd5c1c04
-- 
2.55.0

[PATCH v14 01/12] mm/Kconfig: changes for migrate on fault for device pages

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

HMM depends on MMU notifiers. With the unified HMM/migrate_device
page table walk migrate_device needs HMM enabled.
Enable them explicitly to avoid breaking random configs.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index 604c58199acb..b7a485beb9a7 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -687,6 +687,7 @@ config MIGRATION
 
 config DEVICE_MIGRATION
 	def_bool MIGRATION && ZONE_DEVICE
+	select HMM_MIRROR
 
 config ARCH_ENABLE_HUGEPAGE_MIGRATION
 	bool
-- 
2.55.0

[PATCH v14 02/12] mm: add helper to convert HMM pfn to migrate pfn

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

The unified HMM/migrate_device pagewalk does the "collecting"
on the HMM side, so we need a helper to transfer pfns to the
migrate_vma world.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 include/linux/hmm.h     | 29 +++++++++++++++++-----
 include/linux/migrate.h |  3 ++-
 mm/migrate_device.c     | 55 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 80 insertions(+), 7 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 6f04e3932f5b..4f56f3419cb4 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -13,6 +13,8 @@
 
 struct mmu_interval_notifier;
 
+struct migrate_vma;
+
 /*
  * On output:
  * 0             - The page is faultable and a future call with 
@@ -27,13 +29,22 @@ struct mmu_interval_notifier;
  * HMM_PFN_P2PDMA_BUS - Bus mapped P2P transfer
  * HMM_PFN_DMA_MAPPED - Flag preserved on input-to-output transformation
  *                      to mark that page is already DMA mapped
+ * HMM_PFN_MIGRATE    - The entry is to be migrated. Note, HMM_PFN_MIGRATE
+ *                      alone without HMM_PFN_VALID denotes the
+ *                      empty page.
+ *                      This flag optionally together with HMM_PFN_COMPOUND
+ *                      are indicators for migrate_hmm_range_setup() to
+ *                      setup the migrate pfns.
+ * HMM_PFN_COMPOUND   - The entry represents a > 0 order page
  *
  * On input:
- * 0                 - Return the current state of the page, do not fault it.
- * HMM_PFN_REQ_FAULT - The output must have HMM_PFN_VALID or hmm_range_fault()
- *                     will fail
- * HMM_PFN_REQ_WRITE - The output must have HMM_PFN_WRITE or hmm_range_fault()
- *                     will fail. Must be combined with HMM_PFN_REQ_FAULT.
+ * 0                   - Return the current state of the page, do not fault it.
+ * HMM_PFN_REQ_FAULT   - The output must have HMM_PFN_VALID or hmm_range_fault()
+ *                       will fail
+ * HMM_PFN_REQ_WRITE   - The output must have HMM_PFN_WRITE or hmm_range_fault()
+ *                       will fail. Must be combined with HMM_PFN_REQ_FAULT.
+ * HMM_PFN_REQ_MIGRATE - For default_flags only, request to migrate the range,
+ *                       according to hmm_range.migrate.flags
  */
 enum hmm_pfn_flags {
 	/* Output fields and flags */
@@ -48,11 +59,15 @@ enum hmm_pfn_flags {
 	HMM_PFN_P2PDMA     = 1UL << (BITS_PER_LONG - 5),
 	HMM_PFN_P2PDMA_BUS = 1UL << (BITS_PER_LONG - 6),
 
-	HMM_PFN_ORDER_SHIFT = (BITS_PER_LONG - 11),
+	/* Migrate request */
+	HMM_PFN_MIGRATE    = 1UL << (BITS_PER_LONG - 7),
+	HMM_PFN_COMPOUND   = 1UL << (BITS_PER_LONG - 8),
+	HMM_PFN_ORDER_SHIFT = (BITS_PER_LONG - 13),
 
 	/* Input flags */
 	HMM_PFN_REQ_FAULT = HMM_PFN_VALID,
 	HMM_PFN_REQ_WRITE = HMM_PFN_WRITE,
+	HMM_PFN_REQ_MIGRATE = HMM_PFN_MIGRATE,
 
 	HMM_PFN_FLAGS = ~((1UL << HMM_PFN_ORDER_SHIFT) - 1),
 };
@@ -107,6 +122,7 @@ static inline unsigned int hmm_pfn_to_map_order(unsigned long hmm_pfn)
  * @default_flags: default flags for the range (write, read, ... see hmm doc)
  * @pfn_flags_mask: allows to mask pfn flags so that only default_flags matter
  * @dev_private_owner: owner of device private pages
+ * @migrate: structure for migrating a range of a VMA
  */
 struct hmm_range {
 	struct mmu_interval_notifier *notifier;
@@ -117,6 +133,7 @@ struct hmm_range {
 	unsigned long		default_flags;
 	unsigned long		pfn_flags_mask;
 	void			*dev_private_owner;
+	struct migrate_vma      *migrate;
 };
 
 /*
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 78424b3824c2..280ec9a866c7 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -3,6 +3,7 @@
 #define _LINUX_MIGRATE_H
 
 #include <linux/mm.h>
+#include <linux/hmm.h>
 #include <linux/mempolicy.h>
 #include <linux/migrate_mode.h>
 #include <linux/hugetlb.h>
@@ -202,7 +203,7 @@ void migrate_device_pages(unsigned long *src_pfns, unsigned long *dst_pfns,
 			unsigned long npages);
 void migrate_device_finalize(unsigned long *src_pfns,
 			unsigned long *dst_pfns, unsigned long npages);
-
+void migrate_hmm_range_setup(struct hmm_range *range);
 #endif /* CONFIG_MIGRATION */
 
 #endif /* _LINUX_MIGRATE_H */
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 009bfa8b212d..43e571e82cca 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1528,3 +1528,58 @@ int migrate_device_coherent_folio(struct folio *folio)
 		return 0;
 	return -EBUSY;
 }
+
+/**
+ * migrate_hmm_range_setup() - prepare to migrate a range of memory
+ * @range: contains pointer to struct migrate_vma to be set up.
+ *
+ * When collecting has been done with hmm_range_fault(), this
+ * should be called next, and completes range->migrate by
+ * populating migrate->src[] and migrate->dst[]
+ * using range->hmm_pfns[].
+ * Also, migrate->cpages and migrate->npages get initialized.
+ * After migrate_hmm_range_setup(), range->migrate is good
+ * for the rest of the migrate_vma_* flow.
+ */
+void migrate_hmm_range_setup(struct hmm_range *range)
+{
+	struct migrate_vma *migrate = range->migrate;
+
+	if (!migrate)
+		return;
+
+	migrate->npages = (migrate->end - migrate->start) >> PAGE_SHIFT;
+	migrate->cpages = 0;
+
+	for (unsigned long i = 0; i < migrate->npages; i++) {
+		unsigned long pfn = range->hmm_pfns[i];
+
+		/*
+		 * We are only interested in entries to be
+		 * migrated.
+		 */
+		if (!(pfn & HMM_PFN_MIGRATE)) {
+			migrate->src[i] = 0;
+			migrate->dst[i] = 0;
+			continue;
+		}
+
+		migrate->cpages++;
+
+		/* HMM_PFN_MIGRATE without HMM_PFN_VALID denotes the special zero page */
+		if (pfn & HMM_PFN_VALID)
+			migrate->src[i] = migrate_pfn(page_to_pfn(hmm_pfn_to_page(pfn)));
+		else
+			migrate->src[i] = 0;
+
+		migrate->src[i] |= MIGRATE_PFN_MIGRATE;
+		migrate->src[i] |= (pfn & HMM_PFN_WRITE) ? MIGRATE_PFN_WRITE : 0;
+		migrate->src[i] |= (pfn & HMM_PFN_COMPOUND) ? MIGRATE_PFN_COMPOUND : 0;
+		migrate->dst[i] = 0;
+	}
+
+	if (migrate->cpages)
+		migrate_vma_unmap(migrate);
+
+}
+EXPORT_SYMBOL(migrate_hmm_range_setup);
-- 
2.55.0

[PATCH v14 03/12] mm/hmm: preparations for HMM to participate in migration

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

For migration to happen after hmm_range_fault(), the vma has to
retrieved. Luckily, pagewalk already resolves that for us, so
just have to save it for further use. For migration, mmap_lock
has to be hold for the whole operation, so vma stays stable.
In case lock-drop behavior is requested, return an error while
migrating. Also, rename some struct hmm_vma_walk fields to solve
namespace collisions, and we are later introducing more
xxlocked fields so be unambiguous.

Take care also of firing the mmu_notifier when migrating.

Also, prepare for differencies of how mm_struct * is retrieved
in fault and migrate_vma paths.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 include/linux/migrate.h |  49 +++++++++++++++++-
 lib/test_hmm.c          |   2 +-
 mm/hmm.c                | 107 ++++++++++++++++++++++++++++++++++------
 3 files changed, 141 insertions(+), 17 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 280ec9a866c7..01c5f62a56e9 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -5,6 +5,7 @@
 #include <linux/mm.h>
 #include <linux/hmm.h>
 #include <linux/mempolicy.h>
+#include <linux/mmu_notifier.h>
 #include <linux/migrate_mode.h>
 #include <linux/hugetlb.h>
 
@@ -108,6 +109,29 @@ static inline void softleaf_entry_wait_on_locked(softleaf_t entry, spinlock_t *p
 	spin_unlock(ptl);
 }
 
+enum migrate_vma_info {
+	MIGRATE_VMA_SELECT_NONE = 0,
+	MIGRATE_VMA_SELECT_COMPOUND = MIGRATE_VMA_SELECT_NONE,
+};
+
+static inline enum migrate_vma_info hmm_select_migrate(struct hmm_range *range)
+{
+	return MIGRATE_VMA_SELECT_NONE;
+}
+
+static inline void hmm_fill_migrate_vma(struct hmm_range *range,
+					struct vm_area_struct *vma,
+					unsigned long start,
+					unsigned long end)
+{
+}
+
+#ifdef CONFIG_MMU_NOTIFIER
+static inline struct mm_struct *hmm_range_fault_mm(struct hmm_range *range)
+{
+	return range->notifier->mm;
+}
+#endif
 #endif /* CONFIG_MIGRATION */
 
 #ifdef CONFIG_NUMA_BALANCING
@@ -151,7 +175,7 @@ static inline unsigned long migrate_pfn(unsigned long pfn)
 	return (pfn << MIGRATE_PFN_SHIFT) | MIGRATE_PFN_VALID;
 }
 
-enum migrate_vma_direction {
+enum migrate_vma_info {
 	MIGRATE_VMA_SELECT_SYSTEM = 1 << 0,
 	MIGRATE_VMA_SELECT_DEVICE_PRIVATE = 1 << 1,
 	MIGRATE_VMA_SELECT_DEVICE_COHERENT = 1 << 2,
@@ -193,6 +217,29 @@ struct migrate_vma {
 	struct page		*fault_page;
 };
 
+// TODO: enable migration
+static inline enum migrate_vma_info hmm_select_migrate(struct hmm_range *range)
+{
+	return 0;
+}
+
+#ifdef CONFIG_MMU_NOTIFIER
+static inline struct mm_struct *hmm_range_fault_mm(struct hmm_range *range)
+{
+	return range->notifier ? range->notifier->mm : range->migrate->vma->vm_mm;
+}
+#endif
+
+static inline void hmm_fill_migrate_vma(struct hmm_range *range,
+					struct vm_area_struct *vma,
+					unsigned long start,
+					unsigned long end)
+{
+	range->migrate->vma   = vma;
+	range->migrate->start = start;
+	range->migrate->end   = end;
+}
+
 int migrate_vma_setup(struct migrate_vma *args);
 void migrate_vma_pages(struct migrate_vma *migrate);
 void migrate_vma_finalize(struct migrate_vma *migrate);
diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 6911daa9f854..cd88e8177d0a 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -145,7 +145,7 @@ static bool dmirror_is_private_zone(struct dmirror_device *mdevice)
 		HMM_DMIRROR_MEMORY_DEVICE_PRIVATE);
 }
 
-static enum migrate_vma_direction
+static enum migrate_vma_info
 dmirror_select_device(struct dmirror *dmirror)
 {
 	return (dmirror->mdevice->zone_device_type ==
diff --git a/mm/hmm.c b/mm/hmm.c
index 2f1e98c6b644..be39d2e5403a 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -21,6 +21,7 @@
 #include <linux/pagemap.h>
 #include <linux/leafops.h>
 #include <linux/hugetlb.h>
+#include <linux/migrate.h>
 #include <linux/memremap.h>
 #include <linux/sched/mm.h>
 #include <linux/jump_label.h>
@@ -28,15 +29,21 @@
 #include <linux/pci-p2pdma.h>
 #include <linux/mmu_notifier.h>
 #include <linux/memory_hotplug.h>
+#include <asm/tlbflush.h>
 
 #include "internal.h"
 
 struct hmm_vma_walk {
-	struct hmm_range	*range;
-	bool			*locked;
-	unsigned long		last;
-	unsigned long		end;
-	unsigned int		required_fault;
+	struct mmu_notifier_range	mmu_range;
+	struct vm_area_struct		*vma;
+	struct hmm_range		*range;
+	unsigned long			start;
+	unsigned long			end;
+	unsigned long			last;
+	/* fault and lock drop related fields */
+	bool				*mmlocked;
+	unsigned long			fault_end;
+	unsigned int			required_fault;
 };
 
 /*
@@ -95,7 +102,7 @@ static int hmm_record_fault(unsigned long addr, unsigned long end,
 
 	WARN_ON_ONCE(!required_fault);
 	hmm_vma_walk->last = addr;
-	hmm_vma_walk->end = end;
+	hmm_vma_walk->fault_end = end;
 	hmm_vma_walk->required_fault = required_fault;
 	return HMM_FAULT_PENDING;
 }
@@ -400,6 +407,57 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 }
 #endif  /* CONFIG_ARCH_HAS_PMD_SOFTLEAVES */
 
+static int hmm_vma_capture_migrate_range(unsigned long start,
+					 unsigned long end,
+					 struct mm_walk *walk)
+{
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
+
+	if (!hmm_select_migrate(range))
+		return 0;
+
+	if (hmm_vma_walk->vma && (hmm_vma_walk->vma != walk->vma))
+		return -ERANGE;
+
+	hmm_vma_walk->vma = walk->vma;
+	hmm_vma_walk->start = start;
+	hmm_vma_walk->end = end;
+
+	if (end - start > range->end - range->start)
+		return -ERANGE;
+
+	if (!hmm_vma_walk->mmu_range.owner) {
+		mmu_notifier_range_init_owner(&hmm_vma_walk->mmu_range, MMU_NOTIFY_MIGRATE, 0,
+					      walk->vma->vm_mm, start, end,
+					      range->dev_private_owner);
+		mmu_notifier_invalidate_range_start(&hmm_vma_walk->mmu_range);
+	}
+
+	return 0;
+}
+
+static void hmm_vma_post_range_fault(struct hmm_vma_walk *hmm_vma_walk)
+{
+
+	struct hmm_range *range = hmm_vma_walk->range;
+
+	if (hmm_select_migrate(range) &&
+	    hmm_vma_walk->mmu_range.owner) {
+		/*
+		 *  The migrate_vma path has the following initialized,
+		 *  so take care of fault path below.
+		 */
+		if (range->notifier) {
+			hmm_fill_migrate_vma(range,
+					     hmm_vma_walk->vma,
+					     hmm_vma_walk->start,
+					     hmm_vma_walk->end);
+		}
+		mmu_notifier_invalidate_range_end(&hmm_vma_walk->mmu_range);
+	}
+}
+
 static int hmm_vma_walk_pmd(pmd_t *pmdp,
 			    unsigned long start,
 			    unsigned long end,
@@ -594,6 +652,11 @@ static int hmm_vma_walk_test(unsigned long start, unsigned long end,
 	struct hmm_vma_walk *hmm_vma_walk = walk->private;
 	struct hmm_range *range = hmm_vma_walk->range;
 	struct vm_area_struct *vma = walk->vma;
+	int r;
+
+	r = hmm_vma_capture_migrate_range(start, end, walk);
+	if (r)
+		return r;
 
 	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)) &&
 	    vma->vm_flags & VM_READ)
@@ -646,12 +709,12 @@ static int hmm_do_fault(struct mm_struct *mm,
 			struct hmm_vma_walk *hmm_vma_walk)
 {
 	unsigned long addr = hmm_vma_walk->last;
-	unsigned long end = hmm_vma_walk->end;
+	unsigned long end = hmm_vma_walk->fault_end;
 	unsigned int required_fault = hmm_vma_walk->required_fault;
 	unsigned int fault_flags = FAULT_FLAG_REMOTE;
 	struct vm_area_struct *vma;
 
-	if (hmm_vma_walk->locked)
+	if (hmm_vma_walk->mmlocked)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
 
 	vma = vma_lookup(mm, addr);
@@ -670,8 +733,8 @@ static int hmm_do_fault(struct mm_struct *mm,
 		ret = handle_mm_fault(vma, addr, fault_flags, NULL);
 
 		if (ret & (VM_FAULT_COMPLETED | VM_FAULT_RETRY)) {
-			if (hmm_vma_walk->locked)    /* needed by sparse */
-				*hmm_vma_walk->locked = false;
+			if (hmm_vma_walk->mmlocked)    /* needed by sparse */
+				*hmm_vma_walk->mmlocked = false;
 			else
 				WARN_ON_ONCE(1);    /* broken fault handler */
 			return HMM_FAULT_UNLOCKED;
@@ -694,19 +757,31 @@ static int hmm_range_fault_locked(struct hmm_range *range, bool *locked)
 {
 	struct hmm_vma_walk hmm_vma_walk = {
 		.range = range,
-		.locked = locked,
+		.mmlocked = locked,
 		.last = range->start,
 	};
-	struct mm_struct *mm = range->notifier->mm;
+	/*
+	 *  Could be serving a device fault or come from migrate
+	 *  entry point. For the former we have not resolved the vma
+	 *  yet, and the latter we don't have a notifier (but have a vma).
+	 *
+	 */
+	struct mm_struct *mm = hmm_range_fault_mm(range);
 	int ret;
 
+	/* Migration is incompatible with mmap lock drop */
+	if (locked && hmm_select_migrate(range))
+		return -EINVAL;
+
 	mmap_assert_locked(mm);
 
 	do {
 		/* If range is no longer valid force retry. */
-		if (mmu_interval_check_retry(range->notifier,
-					     range->notifier_seq))
-			return -EBUSY;
+		if (range->notifier && mmu_interval_check_retry(range->notifier,
+								range->notifier_seq)) {
+			ret =  -EBUSY;
+			break;
+		}
 		ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
 				      &hmm_walk_ops, &hmm_vma_walk);
 		/*
@@ -732,6 +807,8 @@ static int hmm_range_fault_locked(struct hmm_range *range, bool *locked)
 		 * output, and all >= are still at their input values.
 		 */
 	} while (ret == -EBUSY);
+
+	hmm_vma_post_range_fault(&hmm_vma_walk);
 	return ret;
 }
 
-- 
2.55.0

[PATCH v14 04/12] mm/hmm: do the plumbing for HMM to participate in migration

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

Do the preparations in hmm_range_fault() and pagewalk callbacks to
do the "collecting" part of migration, needed for migration.

These steps include locking for pmd/pte if migrating, and calling the
still dummy hmm_vma_handle_migrate_prepare_pmd() and
hmm_vma_handle_migrate_prepare()  functions in the pagewalk.

When doing migration, have to have pmd/pte table locked for the
duration of both hmm_vma_handle*() and hmm_vma_handle_migrate_prepare*()
walks, and unlock when handling the faults.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 mm/hmm.c | 361 +++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 310 insertions(+), 51 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index be39d2e5403a..4805c88347e6 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -40,6 +40,16 @@ struct hmm_vma_walk {
 	unsigned long			start;
 	unsigned long			end;
 	unsigned long			last;
+	/*
+	 * For migration we need pte/pmd locked for the handle_* and
+	 * prepare_* regions. While faulting we have to drop the locks
+	 * and start again. ptelocked and pmdlocked hold the state
+	 * and tell  if need to drop locks before faulting.
+	 * ptl is the lock held for pte or pmd.
+	 */
+	bool				ptelocked;
+	bool				pmdlocked;
+	spinlock_t			*ptl;
 	/* fault and lock drop related fields */
 	bool				*mmlocked;
 	unsigned long			fault_end;
@@ -61,6 +71,16 @@ struct hmm_vma_walk {
  */
 #define HMM_FAULT_UNLOCKED	-ENOLCK
 
+#define HMM_ASSERT_PTE_LOCKED(hmm_vma_walk, locked)		\
+	WARN_ON_ONCE((hmm_vma_walk)->ptelocked != locked)
+
+#define HMM_ASSERT_PMD_LOCKED(hmm_vma_walk, locked)		\
+	WARN_ON_ONCE((hmm_vma_walk)->pmdlocked != locked)
+
+#define HMM_ASSERT_UNLOCKED(hmm_vma_walk)			\
+	WARN_ON_ONCE((hmm_vma_walk)->ptelocked ||		\
+		     (hmm_vma_walk)->pmdlocked)
+
 enum {
 	HMM_NEED_FAULT = 1 << 0,
 	HMM_NEED_WRITE_FAULT = 1 << 1,
@@ -74,14 +94,38 @@ enum {
 };
 
 static int hmm_pfns_fill(unsigned long addr, unsigned long end,
-			 struct hmm_range *range, unsigned long cpu_flags)
+			 struct hmm_vma_walk *hmm_vma_walk, unsigned long cpu_flags)
 {
+	struct hmm_range *range = hmm_vma_walk->range;
 	unsigned long i = (addr - range->start) >> PAGE_SHIFT;
+	enum migrate_vma_info minfo;
+	bool migrate = false;
+
+	minfo = hmm_select_migrate(range);
+	if (cpu_flags != HMM_PFN_ERROR) {
+		if (minfo && (vma_is_anonymous(hmm_vma_walk->vma))) {
+			cpu_flags |= HMM_PFN_MIGRATE;
+			migrate = true;
+		}
+	}
+
+	if (migrate && thp_migration_supported() &&
+	    (minfo & MIGRATE_VMA_SELECT_COMPOUND) &&
+	    IS_ALIGNED(addr, HPAGE_PMD_SIZE) &&
+	    IS_ALIGNED(end, HPAGE_PMD_SIZE) &&
+		end-addr == HPAGE_PMD_SIZE) {
+		range->hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS;
+		range->hmm_pfns[i] |= cpu_flags | HMM_PFN_COMPOUND;
+		addr += PAGE_SIZE;
+		i++;
+		cpu_flags = 0;
+	}
 
 	for (; addr < end; addr += PAGE_SIZE, i++) {
 		range->hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS;
 		range->hmm_pfns[i] |= cpu_flags;
 	}
+
 	return 0;
 }
 
@@ -101,6 +145,7 @@ static int hmm_record_fault(unsigned long addr, unsigned long end,
 	struct hmm_vma_walk *hmm_vma_walk = walk->private;
 
 	WARN_ON_ONCE(!required_fault);
+	HMM_ASSERT_UNLOCKED(hmm_vma_walk);
 	hmm_vma_walk->last = addr;
 	hmm_vma_walk->fault_end = end;
 	hmm_vma_walk->required_fault = required_fault;
@@ -185,11 +230,16 @@ static int hmm_vma_walk_hole(unsigned long addr, unsigned long end,
 	if (!walk->vma) {
 		if (required_fault)
 			return -EFAULT;
-		return hmm_pfns_fill(addr, end, range, HMM_PFN_ERROR);
+		return hmm_pfns_fill(addr, end, hmm_vma_walk, HMM_PFN_ERROR);
 	}
-	if (required_fault)
+	if (required_fault) {
+		if (hmm_vma_walk->pmdlocked) {
+			spin_unlock(hmm_vma_walk->ptl);
+			hmm_vma_walk->pmdlocked = false;
+		}
 		return hmm_record_fault(addr, end, required_fault, walk);
-	return hmm_pfns_fill(addr, end, range, 0);
+	}
+	return hmm_pfns_fill(addr, end, hmm_vma_walk, 0);
 }
 
 static inline unsigned long hmm_pfn_flags_order(unsigned long order)
@@ -222,8 +272,13 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 	cpu_flags = pmd_to_hmm_pfn_flags(range, pmd);
 	required_fault =
 		hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, cpu_flags);
-	if (required_fault)
+	if (required_fault) {
+		if (hmm_vma_walk->pmdlocked) {
+			spin_unlock(hmm_vma_walk->ptl);
+			hmm_vma_walk->pmdlocked = false;
+		}
 		return hmm_record_fault(addr, end, required_fault, walk);
+	}
 
 	pfn = pmd_pfn(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
@@ -303,14 +358,25 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 			goto fault;
 
 		if (softleaf_is_migration(entry)) {
-			pte_unmap(ptep);
-			hmm_vma_walk->last = addr;
-			migration_entry_wait(walk->mm, pmdp, addr);
-			return -EBUSY;
+			if (!hmm_select_migrate(range)) {
+				HMM_ASSERT_UNLOCKED(hmm_vma_walk);
+				pte_unmap(ptep);
+				hmm_vma_walk->last = addr;
+				migration_entry_wait(walk->mm, pmdp, addr);
+				return -EBUSY;
+			}
+			return 0;
 		}
 
 		/* Report error for everything else */
-		pte_unmap(ptep);
+
+		if (hmm_vma_walk->ptelocked) {
+			pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
+			hmm_vma_walk->ptelocked = false;
+		} else {
+			pte_unmap(ptep);
+		}
+
 		return -EFAULT;
 	}
 
@@ -327,7 +393,13 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 	if (!vm_normal_page(walk->vma, addr, pte) &&
 	    !is_zero_pfn(pte_pfn(pte))) {
 		if (hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0)) {
-			pte_unmap(ptep);
+			if (hmm_vma_walk->ptelocked) {
+				pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
+				hmm_vma_walk->ptelocked = false;
+			} else {
+				pte_unmap(ptep);
+			}
+
 			return -EFAULT;
 		}
 		new_pfn_flags = HMM_PFN_ERROR;
@@ -340,7 +412,12 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 	return 0;
 
 fault:
-	pte_unmap(ptep);
+	if (hmm_vma_walk->ptelocked) {
+		pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
+		hmm_vma_walk->ptelocked = false;
+	} else {
+		pte_unmap(ptep);
+	}
 	/* Fault any virtual address we were asked to fault */
 	return hmm_record_fault(addr, end, required_fault, walk);
 }
@@ -384,13 +461,18 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 	required_fault = hmm_range_need_fault(hmm_vma_walk, hmm_pfns,
 					      npages, 0);
 	if (required_fault) {
-		if (softleaf_is_device_private(entry))
+		if (softleaf_is_device_private(entry)) {
+			if (hmm_vma_walk->pmdlocked) {
+				spin_unlock(hmm_vma_walk->ptl);
+				hmm_vma_walk->pmdlocked = false;
+			}
 			return hmm_record_fault(addr, end, required_fault, walk);
+		}
 		else
 			return -EFAULT;
 	}
 
-	return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
+	return hmm_pfns_fill(start, end, hmm_vma_walk, HMM_PFN_ERROR);
 }
 #else
 static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
@@ -398,15 +480,72 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 				     pmd_t pmd)
 {
 	struct hmm_vma_walk *hmm_vma_walk = walk->private;
-	struct hmm_range *range = hmm_vma_walk->range;
 	unsigned long npages = (end - start) >> PAGE_SHIFT;
 
 	if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
 		return -EFAULT;
-	return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
+	return hmm_pfns_fill(start, end, hmm_vma_walk, HMM_PFN_ERROR);
 }
 #endif  /* CONFIG_ARCH_HAS_PMD_SOFTLEAVES */
 
+#ifdef CONFIG_DEVICE_MIGRATION
+static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
+					      pmd_t *pmdp,
+					      unsigned long start,
+					      unsigned long end,
+					      unsigned long *hmm_pfn)
+{
+	// TODO: implement migration entry insertion
+	return 0;
+}
+
+static int hmm_vma_handle_migrate_prepare(const struct mm_walk *walk,
+					  pmd_t *pmdp,
+					  pte_t *ptep,
+					  unsigned long addr,
+					  unsigned long *hmm_pfn,
+					  bool *unmapped)
+{
+	// TODO: implement migration entry insertion
+	return 0;
+}
+
+static int hmm_vma_walk_split(pmd_t *pmdp,
+			      unsigned long addr,
+			      struct mm_walk *walk)
+{
+	// TODO : implement split
+	return 0;
+}
+
+#else
+static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
+					      pmd_t *pmdp,
+					      unsigned long start,
+					      unsigned long end,
+					      unsigned long *hmm_pfn)
+{
+	return 0;
+}
+
+static int hmm_vma_handle_migrate_prepare(const struct mm_walk *walk,
+					  pmd_t *pmdp,
+					  pte_t *ptep,
+					  unsigned long addr,
+					  unsigned long *hmm_pfn,
+					  bool *unmapped)
+{
+	return 0;
+}
+
+static int hmm_vma_walk_split(pmd_t *pmdp,
+			      unsigned long addr,
+			      struct mm_walk *walk)
+{
+	return 0;
+}
+#endif
+
 static int hmm_vma_capture_migrate_range(unsigned long start,
 					 unsigned long end,
 					 struct mm_walk *walk)
@@ -465,46 +604,128 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 {
 	struct hmm_vma_walk *hmm_vma_walk = walk->private;
 	struct hmm_range *range = hmm_vma_walk->range;
-	unsigned long *hmm_pfns =
-		&range->hmm_pfns[(start - range->start) >> PAGE_SHIFT];
 	unsigned long npages = (end - start) >> PAGE_SHIFT;
+	struct mm_struct *mm = walk->vma->vm_mm;
+	unsigned long *hmm_pfns, *hmm_pfns_start;
+	enum migrate_vma_info minfo;
 	unsigned long addr = start;
+	bool unmapped = false;
+	unsigned long i;
 	pte_t *ptep;
 	pmd_t pmd;
+	int r = 0;
 
+	minfo = hmm_select_migrate(range);
+	hmm_pfns_start = &range->hmm_pfns[(start - range->start) >> PAGE_SHIFT];
 again:
-	pmd = pmdp_get_lockless(pmdp);
-	if (pmd_none(pmd))
-		return hmm_vma_walk_hole(start, end, -1, walk);
+	hmm_pfns = &range->hmm_pfns[(addr - range->start) >> PAGE_SHIFT];
+	hmm_vma_walk->ptelocked = false;
+	hmm_vma_walk->pmdlocked = false;
+
+	if (minfo) {
+		hmm_vma_walk->ptl = pmd_lock(mm, pmdp);
+		hmm_vma_walk->pmdlocked = true;
+		pmd = pmdp_get(pmdp);
+	} else
+		pmd = pmdp_get_lockless(pmdp);
+
+	if (pmd_none(pmd)) {
+		r = hmm_vma_walk_hole(start, end, -1, walk);
+
+		if (hmm_vma_walk->pmdlocked) {
+			spin_unlock(hmm_vma_walk->ptl);
+			hmm_vma_walk->pmdlocked = false;
+		}
+		return r;
+	}
 
 	if (thp_migration_supported() && pmd_is_migration_entry(pmd)) {
-		if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0)) {
-			hmm_vma_walk->last = addr;
-			pmd_migration_entry_wait(walk->mm, pmdp);
-			return -EBUSY;
+		if (!minfo) {
+			if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns_start, npages, 0)) {
+				hmm_vma_walk->last = addr;
+				pmd_migration_entry_wait(walk->mm, pmdp);
+				return -EBUSY;
+			}
+			for (i = 0; start < end; start += PAGE_SIZE, i++)
+				hmm_pfns_start[i] &= HMM_PFN_INOUT_FLAGS;
+		}
+
+		if (hmm_vma_walk->pmdlocked) {
+			spin_unlock(hmm_vma_walk->ptl);
+			hmm_vma_walk->pmdlocked = false;
 		}
-		return hmm_pfns_fill(start, end, range, 0);
+
+		return 0;
 	}
 
-	if (!pmd_present(pmd))
-		return hmm_vma_handle_absent_pmd(walk, start, end, hmm_pfns,
-						 pmd);
+	if (pmd_trans_huge(pmd) || !pmd_present(pmd)) {
+		if (!pmd_present(pmd)) {
+			r = hmm_vma_handle_absent_pmd(walk, start, end, hmm_pfns_start,
+						      pmd);
+			// If not migrating we are done
+			if (r || !minfo) {
+				if (hmm_vma_walk->pmdlocked) {
+					spin_unlock(hmm_vma_walk->ptl);
+					hmm_vma_walk->pmdlocked = false;
+				}
+				return r;
+			}
+		}
 
-	if (pmd_trans_huge(pmd)) {
-		/*
-		 * No need to take pmd_lock here, even if some other thread
-		 * is splitting the huge pmd we will get that event through
-		 * mmu_notifier callback.
-		 *
-		 * So just read pmd value and check again it's a transparent
-		 * huge or device mapping one and compute corresponding pfn
-		 * values.
-		 */
-		pmd = pmdp_get_lockless(pmdp);
-		if (!pmd_trans_huge(pmd))
-			goto again;
+		if (pmd_trans_huge(pmd)) {
+			/*
+			 * No need to take pmd_lock here if not migrating,
+			 * even if some other thread is splitting the huge
+			 * pmd we will get that event through mmu_notifier callback.
+			 *
+			 * So just read pmd value and check again it's a transparent
+			 * huge or device mapping one and compute corresponding pfn
+			 * values.
+			 */
+
+			if (!minfo) {
+				pmd = pmdp_get_lockless(pmdp);
+				if (!pmd_trans_huge(pmd))
+					goto again;
+			}
+
+			r = hmm_vma_handle_pmd(walk, start, end, hmm_pfns_start, pmd);
 
-		return hmm_vma_handle_pmd(walk, addr, end, hmm_pfns, pmd);
+			// If not migrating we are done
+			if (r || !minfo) {
+				if (hmm_vma_walk->pmdlocked) {
+					spin_unlock(hmm_vma_walk->ptl);
+					hmm_vma_walk->pmdlocked = false;
+				}
+				return r;
+			}
+		}
+
+		r = hmm_vma_handle_migrate_prepare_pmd(walk, pmdp, start, end, hmm_pfns_start);
+
+		if (hmm_vma_walk->pmdlocked) {
+			spin_unlock(hmm_vma_walk->ptl);
+			hmm_vma_walk->pmdlocked = false;
+		}
+
+		if (r == -ENOENT) {
+			r = hmm_vma_walk_split(pmdp, addr, walk);
+			if (r) {
+				/* Split not successful, skip */
+				return hmm_pfns_fill(start, end, hmm_vma_walk, HMM_PFN_ERROR);
+			}
+
+			/* Split successful, reloop */
+			hmm_vma_walk->last = addr;
+			return -EBUSY;
+		}
+		return r;
+
+	}
+
+	if (hmm_vma_walk->pmdlocked) {
+		spin_unlock(hmm_vma_walk->ptl);
+		hmm_vma_walk->pmdlocked = false;
 	}
 
 	/*
@@ -514,24 +735,62 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 	 * recover.
 	 */
 	if (pmd_bad(pmd)) {
-		if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
+		if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns_start, npages, 0))
 			return -EFAULT;
-		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
+		return hmm_pfns_fill(start, end, hmm_vma_walk, HMM_PFN_ERROR);
 	}
 
-	ptep = pte_offset_map(pmdp, addr);
-	if (!ptep)
+	if (minfo) {
+		ptep = pte_offset_map_lock(mm, pmdp, addr, &hmm_vma_walk->ptl);
+		if (ptep)
+			hmm_vma_walk->ptelocked = true;
+	} else {
+		ptep = pte_offset_map(pmdp, addr);
+	}
+	if (!ptep) {
+		addr = start;
 		goto again;
+	}
+
 	for (; addr < end; addr += PAGE_SIZE, ptep++, hmm_pfns++) {
-		int r;
 
 		r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, hmm_pfns);
 		if (r) {
-			/* hmm_vma_handle_pte() did pte_unmap() */
+			/* hmm_vma_handle_pte() did pte_unmap() / pte_unmap_unlock */
 			return r;
 		}
+
+		r = hmm_vma_handle_migrate_prepare(walk, pmdp, ptep, addr, hmm_pfns, &unmapped);
+		if (r == -EAGAIN) {
+			HMM_ASSERT_UNLOCKED(hmm_vma_walk);
+			if (unmapped) {
+				flush_tlb_range(walk->vma, start, addr);
+				unmapped = false;
+			}
+			goto again;
+		}
+		if (r) {
+			/* A non -EAGAIN error here means migrate_vma_split_folio()
+			 * already dropped the PTE lock and cleared ptelocked.
+			 */
+			HMM_ASSERT_UNLOCKED(hmm_vma_walk);
+			if (unmapped)
+				flush_tlb_range(walk->vma, start, addr);
+			hmm_pfns_fill(addr, end, hmm_vma_walk, HMM_PFN_ERROR);
+			return 0;
+		}
 	}
-	pte_unmap(ptep - 1);
+
+	if (unmapped)
+		flush_tlb_range(walk->vma, start, addr);
+
+	if (hmm_vma_walk->ptelocked) {
+		pte_unmap_unlock(ptep - 1, hmm_vma_walk->ptl);
+		hmm_vma_walk->ptelocked = false;
+	} else {
+		pte_unmap(ptep - 1);
+	}
+
 	return 0;
 }
 
@@ -679,7 +938,7 @@ static int hmm_vma_walk_test(unsigned long start, unsigned long end,
 				 (end - start) >> PAGE_SHIFT, 0))
 		return -EFAULT;
 
-	hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
+	hmm_pfns_fill(start, end, hmm_vma_walk, HMM_PFN_ERROR);
 
 	/* Skip this vma and continue processing the next vma. */
 	return 1;
-- 
2.55.0

[PATCH v14 05/12] mm/hmm: implement folio split for migrate needs in HMM pagewalk

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

Implement the helper to split PMD size folios during pagewalk.
Splitting is needed if the start and end addresses are
not PMD aligned, or setting up pmd migration entry fails.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 mm/hmm.c | 47 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 4805c88347e6..e6469ef4ae6a 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -514,10 +514,51 @@ static int hmm_vma_walk_split(pmd_t *pmdp,
 			      unsigned long addr,
 			      struct mm_walk *walk)
 {
-	// TODO : implement split
-	return 0;
-}
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
+	struct migrate_vma *migrate = range->migrate;
+	struct folio *folio, *fault_folio;
+	spinlock_t *ptl;
+	int ret = 0;
+
+	HMM_ASSERT_UNLOCKED(hmm_vma_walk);
+
+	fault_folio = (migrate && migrate->fault_page) ?
+		page_folio(migrate->fault_page) : NULL;
+
+	ptl = pmd_lock(walk->mm, pmdp);
+	if (unlikely(!pmd_trans_huge(*pmdp))) {
+		spin_unlock(ptl);
+		goto out;
+	}
 
+	folio = pmd_folio(*pmdp);
+	if (is_huge_zero_folio(folio)) {
+		spin_unlock(ptl);
+		split_huge_pmd(walk->vma, pmdp, addr);
+	} else {
+		folio_get(folio);
+		spin_unlock(ptl);
+
+		if (folio != fault_folio) {
+			if (unlikely(!folio_trylock(folio))) {
+				folio_put(folio);
+				ret = -EBUSY;
+				goto out;
+			}
+		}  else {
+			folio_put(folio);
+		}
+
+		ret = split_folio(folio);
+		if (fault_folio != folio) {
+			folio_unlock(folio);
+			folio_put(folio);
+		}
+	}
+out:
+	return ret;
+}
 #else
 static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
 					      pmd_t *pmdp,
-- 
2.55.0

[PATCH v14 06/12] mm/hmm: migrate collection in HMM pagewalk - pte level

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

Implement the needed hmm_vma_handle_migrate_prepare() function
which is mostly carried over from migrate_device.c's
migrate_vma_collect_pmd() function.

Also implement the migrate_vma_split_folio(), for splitting
pte mapped large folios. It is also mostly from migrate_device.c,
with care taken to reference folio before relasing page table
lock.

With HMM pagewalk based migration, the idea is that
hmm_vma_handle_*() are responsible for faulting,
and the pfn collecting part. hmm_vma_handle_migrate_prepare*()
do the migration decisions (with HMM_PFN_MIGRATE), possibly split
folios, and insert migration ptes/pmds.

HMM pagewalk based migration is enabled in later commit, for now
now hmm_select_migrate() returns 0.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 mm/hmm.c | 262 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 261 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index e6469ef4ae6a..b3b79d13c797 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -489,6 +489,63 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 #endif  /* CONFIG_ARCH_HAS_PMD_SOFTLEAVES */
 
 #ifdef CONFIG_DEVICE_MIGRATION
+/**
+ * migrate_vma_split_folio() - Helper function to split a THP folio
+ * @folio: the folio to split
+ * @fault_page: struct page associated with the fault if any
+ * @hmm_vma_walk: walk in progress
+ * @ptep: pte_t * for unmap and unlock ptl
+ *
+ * Returns 0 on success
+ */
+static int migrate_vma_split_folio(struct folio *folio,
+				   struct page *fault_page,
+				   struct hmm_vma_walk *hmm_vma_walk,
+				   pte_t *ptep)
+{
+	int ret;
+	struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL;
+	struct folio *new_fault_folio = NULL;
+
+	if (folio != fault_folio)
+		folio_get(folio);
+
+	pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
+	hmm_vma_walk->ptelocked = false;
+
+	if (folio != fault_folio)
+		folio_lock(folio);
+
+	ret = split_folio(folio);
+	if (ret) {
+		if (folio != fault_folio) {
+			folio_unlock(folio);
+			folio_put(folio);
+		}
+		return ret;
+	}
+
+	new_fault_folio = fault_page ? page_folio(fault_page) : NULL;
+
+	/*
+	 * Ensure the lock is held on the correct
+	 * folio after the split
+	 */
+	if (!new_fault_folio) {
+		folio_unlock(folio);
+		folio_put(folio);
+	} else if (folio != new_fault_folio) {
+		if (new_fault_folio != fault_folio) {
+			folio_get(new_fault_folio);
+			folio_lock(new_fault_folio);
+		}
+		folio_unlock(folio);
+		folio_put(folio);
+	}
+
+	return 0;
+}
+
 static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
 					      pmd_t *pmdp,
 					      unsigned long start,
@@ -499,6 +556,11 @@ static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
 	return 0;
 }
 
+/*
+ * Install migration entries if migration requested, either from fault
+ * or migrate paths.
+ *
+ */
 static int hmm_vma_handle_migrate_prepare(const struct mm_walk *walk,
 					  pmd_t *pmdp,
 					  pte_t *ptep,
@@ -506,8 +568,206 @@ static int hmm_vma_handle_migrate_prepare(const struct mm_walk *walk,
 					  unsigned long *hmm_pfn,
 					  bool *unmapped)
 {
-	// TODO: implement migration entry insertion
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
+	struct migrate_vma *migrate = range->migrate;
+	struct mm_struct *mm = walk->vma->vm_mm;
+	struct folio *fault_folio = NULL;
+	enum migrate_vma_info minfo;
+	struct dev_pagemap *pgmap;
+	bool anon_exclusive;
+	struct folio *folio;
+	unsigned long pfn;
+	struct page *page;
+	softleaf_t entry;
+	pte_t pte, swp_pte;
+	bool writable = false;
+
+	// Do we want to migrate at all?
+	minfo = hmm_select_migrate(range);
+	if (!minfo)
+		return 0;
+
+	WARN_ON_ONCE(!migrate);
+	HMM_ASSERT_PTE_LOCKED(hmm_vma_walk, true);
+
+	fault_folio = migrate->fault_page ?
+		page_folio(migrate->fault_page) : NULL;
+
+	pte = ptep_get(ptep);
+
+	if (pte_none(pte)) {
+		if (vma_is_anonymous(walk->vma)) {
+			*hmm_pfn &= HMM_PFN_INOUT_FLAGS;
+			*hmm_pfn |= HMM_PFN_MIGRATE;
+			goto out;
+		}
+	}
+
+	if (!(hmm_pfn[0] & HMM_PFN_VALID))
+		goto out;
+
+	if (!pte_present(pte)) {
+		/*
+		 * Only care about unaddressable device page special
+		 * page table entry. Other special swap entries are not
+		 * migratable, and we ignore regular swapped page.
+		 */
+		entry = softleaf_from_pte(pte);
+		if (!softleaf_is_device_private(entry))
+			goto out;
+
+		if (!(minfo & MIGRATE_VMA_SELECT_DEVICE_PRIVATE))
+			goto out;
+
+		page = softleaf_to_page(entry);
+		folio = page_folio(page);
+		if (folio->pgmap->owner != migrate->pgmap_owner)
+			goto out;
+
+		if (folio_test_large(folio)) {
+			int ret;
+
+			ret = migrate_vma_split_folio(folio,
+						      migrate->fault_page,
+						      hmm_vma_walk,
+						      ptep);
+			if (ret)
+				goto out_error;
+			return -EAGAIN;
+		}
+
+		pfn = page_to_pfn(page);
+		if (softleaf_is_device_private_write(entry))
+			writable = true;
+	} else {
+		pfn = pte_pfn(pte);
+		if (is_zero_pfn(pfn) &&
+		    (minfo & MIGRATE_VMA_SELECT_SYSTEM)) {
+			*hmm_pfn = HMM_PFN_MIGRATE;
+			goto out;
+		}
+		page = vm_normal_page(walk->vma, addr, pte);
+		if (page && !is_zone_device_page(page) &&
+		    !(minfo & MIGRATE_VMA_SELECT_SYSTEM)) {
+			goto out;
+		} else if (page && is_device_coherent_page(page)) {
+			pgmap = page_pgmap(page);
+
+			if (!(minfo &
+			      MIGRATE_VMA_SELECT_DEVICE_COHERENT) ||
+			    pgmap->owner != migrate->pgmap_owner)
+				goto out;
+		}
+
+		folio = page ? page_folio(page) : NULL;
+		if (folio && folio_test_large(folio)) {
+			int ret;
+
+			ret = migrate_vma_split_folio(folio,
+						      migrate->fault_page,
+						      hmm_vma_walk,
+						      ptep);
+			if (ret)
+				goto out_error;
+			return -EAGAIN;
+		}
+
+		writable = pte_write(pte);
+	}
+
+	if (!page || !page->mapping)
+		goto out;
+
+	/*
+	 * By getting a reference on the folio we pin it and that blocks
+	 * any kind of migration. Side effect is that it "freezes" the
+	 * pte.
+	 *
+	 * We drop this reference after isolating the folio from the lru
+	 * for non device folio (device folio are not on the lru and thus
+	 * can't be dropped from it).
+	 */
+	folio = page_folio(page);
+	folio_get(folio);
+
+	/*
+	 * We rely on folio_trylock() to avoid deadlock between
+	 * concurrent migrations where each is waiting on the others
+	 * folio lock. If we can't immediately lock the folio we fail this
+	 * migration as it is only best effort anyway.
+	 *
+	 * If we can lock the folio it's safe to set up a migration entry
+	 * now. In the common case where the folio is mapped once in a
+	 * single process setting up the migration entry now is an
+	 * optimisation to avoid walking the rmap later with
+	 * try_to_migrate().
+	 */
+
+	if (fault_folio == folio || folio_trylock(folio)) {
+		anon_exclusive = folio_test_anon(folio) &&
+			PageAnonExclusive(page);
+
+		if (pte_present(pte))
+			flush_cache_page(walk->vma, addr, pfn);
+
+		if (anon_exclusive) {
+			pte = ptep_clear_flush(walk->vma, addr, ptep);
+
+			if (folio_try_share_anon_rmap_pte(folio, page)) {
+				set_pte_at(mm, addr, ptep, pte);
+				folio_unlock(folio);
+				folio_put(folio);
+				goto out;
+			}
+		} else {
+			pte = ptep_get_and_clear(mm, addr, ptep);
+		}
+
+		if (pte_present(pte) && pte_dirty(pte))
+			folio_mark_dirty(folio);
+
+		/* Setup special migration page table entry */
+		if (writable)
+			entry = make_writable_migration_entry(pfn);
+		else if (anon_exclusive)
+			entry = make_readable_exclusive_migration_entry(pfn);
+		else
+			entry = make_readable_migration_entry(pfn);
+
+		if (pte_present(pte)) {
+			if (pte_young(pte))
+				entry = make_migration_entry_young(entry);
+			if (pte_dirty(pte))
+				entry = make_migration_entry_dirty(entry);
+		}
+
+		swp_pte = swp_entry_to_pte(entry);
+		if (pte_present(pte)) {
+			if (pte_soft_dirty(pte))
+				swp_pte = pte_swp_mksoft_dirty(swp_pte);
+			if (pte_uffd(pte))
+				swp_pte = pte_swp_mkuffd(swp_pte);
+		} else {
+			if (pte_swp_soft_dirty(pte))
+				swp_pte = pte_swp_mksoft_dirty(swp_pte);
+			if (pte_swp_uffd(pte))
+				swp_pte = pte_swp_mkuffd(swp_pte);
+		}
+
+		set_pte_at(mm, addr, ptep, swp_pte);
+		folio_remove_rmap_pte(folio, page, walk->vma);
+		folio_put(folio);
+		*hmm_pfn |= HMM_PFN_MIGRATE;
+		if (pte_present(pte))
+			*unmapped = true;
+	} else {
+		folio_put(folio);
+	}
+out:
 	return 0;
+out_error:
+	return -EFAULT;
 }
 
 static int hmm_vma_walk_split(pmd_t *pmdp,
-- 
2.55.0

[PATCH v14 07/12] mm/hmm: migrate collection in HMM pagewalk - pmd level

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

Implement the needed hmm_vma_handle_migrate_prepare_pmd() function
which is mostly carried over from migrate_device.c's
migrate_vma_collect_huge_pmd() function.

With HMM pagewalk based migration, the idea is that
hmm_vma_handle_*() are responsible for faulting,
and the pfn collecting part. hmm_vma_handle_migrate_prepare*()
do the migration decisions (with HMM_PFN_MIGRATE), possibly split
folios, and insert migration ptes/pmds.

HMM pagewalk based migration is enabled in later commit, for now
now hmm_select_migrate() returns 0.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 mm/hmm.c | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 94 insertions(+), 2 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index b3b79d13c797..a1459bd34c04 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -552,8 +552,100 @@ static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
 					      unsigned long end,
 					      unsigned long *hmm_pfn)
 {
-	// TODO: implement migration entry insertion
-	return 0;
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
+	struct migrate_vma *migrate = range->migrate;
+	struct folio *fault_folio = NULL;
+	enum migrate_vma_info minfo;
+	struct folio *folio;
+	unsigned long i;
+	int r = 0;
+
+	// Do we want to migrate at all?
+	minfo = hmm_select_migrate(range);
+	if (!minfo)
+		return r;
+
+	WARN_ON_ONCE(!migrate);
+	HMM_ASSERT_PMD_LOCKED(hmm_vma_walk, true);
+
+	fault_folio = migrate->fault_page ?
+		page_folio(migrate->fault_page) : NULL;
+
+	if (pmd_none(*pmdp))
+		return hmm_pfns_fill(start, end, hmm_vma_walk, 0);
+
+	if (!(hmm_pfn[0] & HMM_PFN_VALID))
+		goto out;
+
+	if (pmd_trans_huge(*pmdp)) {
+		if (!(minfo & MIGRATE_VMA_SELECT_SYSTEM))
+			goto out;
+
+		folio = pmd_folio(*pmdp);
+		if (is_huge_zero_folio(folio))
+			return hmm_pfns_fill(start, end, hmm_vma_walk, 0);
+
+	} else if (!pmd_present(*pmdp)) {
+		const softleaf_t entry = softleaf_from_pmd(*pmdp);
+
+		if (!softleaf_is_device_private(entry))
+			goto out;
+
+		if (!(minfo & MIGRATE_VMA_SELECT_DEVICE_PRIVATE))
+			goto out;
+
+		folio = softleaf_to_folio(entry);
+		if (folio->pgmap->owner != migrate->pgmap_owner)
+			goto out;
+	} else {
+		hmm_vma_walk->last = start;
+		return -EBUSY;
+	}
+
+	folio_get(folio);
+
+	if (folio != fault_folio && unlikely(!folio_trylock(folio))) {
+		folio_put(folio);
+		hmm_pfns_fill(start, end, hmm_vma_walk, HMM_PFN_ERROR);
+		return 0;
+	}
+
+	if (thp_migration_supported() &&
+	    (migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
+	    (IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
+	     IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
+		struct page_vma_mapped_walk pvmw = {
+			.ptl = hmm_vma_walk->ptl,
+			.address = start,
+			.pmd = pmdp,
+			.vma = walk->vma,
+		};
+
+		hmm_pfn[0] |= HMM_PFN_MIGRATE | HMM_PFN_COMPOUND;
+
+		r = set_pmd_migration_entry(&pvmw, folio_page(folio, 0));
+		if (r) {
+			hmm_pfn[0] &= ~(HMM_PFN_MIGRATE | HMM_PFN_COMPOUND);
+			r = -ENOENT;  // fallback
+			goto unlock_out;
+		}
+		for (i = 1, start += PAGE_SIZE; start < end; start += PAGE_SIZE, i++)
+			hmm_pfn[i] &= HMM_PFN_INOUT_FLAGS;
+
+	} else {
+		r = -ENOENT;  // fallback
+		goto unlock_out;
+	}
+
+out:
+	return r;
+
+unlock_out:
+	if (folio != fault_folio)
+		folio_unlock(folio);
+	folio_put(folio);
+	goto out;
 }
 
 /*
-- 
2.55.0

[PATCH v14 08/12] mm/hmm: add lazy MMU mode support for migration in HMM pagewalk

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

Add calls to lazy_mmu_mode_enable() and lazy_mmu_mode_disable()
while doing migration.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 mm/hmm.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index a1459bd34c04..a67a23e54551 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -371,6 +371,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 		/* Report error for everything else */
 
 		if (hmm_vma_walk->ptelocked) {
+			lazy_mmu_mode_disable();
 			pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
 			hmm_vma_walk->ptelocked = false;
 		} else {
@@ -394,6 +395,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 	    !is_zero_pfn(pte_pfn(pte))) {
 		if (hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0)) {
 			if (hmm_vma_walk->ptelocked) {
+				lazy_mmu_mode_disable();
 				pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
 				hmm_vma_walk->ptelocked = false;
 			} else {
@@ -413,6 +415,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 
 fault:
 	if (hmm_vma_walk->ptelocked) {
+		lazy_mmu_mode_disable();
 		pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
 		hmm_vma_walk->ptelocked = false;
 	} else {
@@ -510,6 +513,7 @@ static int migrate_vma_split_folio(struct folio *folio,
 	if (folio != fault_folio)
 		folio_get(folio);
 
+	lazy_mmu_mode_disable();
 	pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
 	hmm_vma_walk->ptelocked = false;
 
@@ -1135,8 +1139,10 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 
 	if (minfo) {
 		ptep = pte_offset_map_lock(mm, pmdp, addr, &hmm_vma_walk->ptl);
-		if (ptep)
+		if (ptep) {
+			lazy_mmu_mode_enable();
 			hmm_vma_walk->ptelocked = true;
+		}
 	} else {
 		ptep = pte_offset_map(pmdp, addr);
 	}
@@ -1178,6 +1184,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 		flush_tlb_range(walk->vma, start, addr);
 
 	if (hmm_vma_walk->ptelocked) {
+		lazy_mmu_mode_disable();
 		pte_unmap_unlock(ptep - 1, hmm_vma_walk->ptl);
 		hmm_vma_walk->ptelocked = false;
 	} else {
-- 
2.55.0

[PATCH v14 09/12] mm/hmm: implement rollback for device page migration in HMM pagewalk

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

During the migration pagewalk, the PTE table could be cleared
and/or changed into PMD leaf or even another PTE table while
dropped locks.

In these cases the possibly inserted migration ptes are gone.
We have to however undo the collecting done so far, so unlock
the folios and drop reference taken.

During the pagewalk we notice such scenarios if going to
recollect a pfn but have already committed to migrate the entry
with HMM_PFN_MIGRATE, in which case rollback.
If we encounter migration ptes they are just skipped to
allow for restart own walks.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 include/linux/hmm.h | 22 +++++++++++++
 mm/hmm.c            | 76 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 98 insertions(+)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 4f56f3419cb4..b08ebc1343dd 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -111,6 +111,28 @@ static inline unsigned int hmm_pfn_to_map_order(unsigned long hmm_pfn)
 	return (hmm_pfn >> HMM_PFN_ORDER_SHIFT) & 0x1F;
 }
 
+/*
+ * hmm_pfn_collected() - is this pfn entry prepared for migration ?
+ * If collected the folio's refcount is increased and the folio
+ * is locked.
+ */
+static inline bool hmm_pfn_collected(unsigned long hmm_pfn)
+{
+	return (hmm_pfn & (HMM_PFN_VALID | HMM_PFN_MIGRATE)) ==
+		(HMM_PFN_VALID | HMM_PFN_MIGRATE);
+}
+
+/*
+ * hmm_pfn_rollback_collected() - undoes the collection of hmm_pfn
+ *
+ * Note for total rollback the folio's refcount has to be put
+ * and folio has to be unlocked.
+ */
+static inline unsigned long hmm_pfn_rollback_collected(unsigned long hmm_pfn)
+{
+	return hmm_pfn & ~(HMM_PFN_VALID | HMM_PFN_MIGRATE | HMM_PFN_COMPOUND);
+}
+
 /*
  * struct hmm_range - track invalidation lock on virtual address range
  *
diff --git a/mm/hmm.c b/mm/hmm.c
index a67a23e54551..e64444cbc62a 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -93,6 +93,11 @@ enum {
 			      HMM_PFN_P2PDMA_BUS,
 };
 
+static void hmm_vma_handle_migrate_prepare_rollback(const struct hmm_vma_walk *hmm_vma_walk,
+						    unsigned long start,
+						    unsigned long end,
+						    unsigned long *hmm_pfn);
+
 static int hmm_pfns_fill(unsigned long addr, unsigned long end,
 			 struct hmm_vma_walk *hmm_vma_walk, unsigned long cpu_flags)
 {
@@ -109,6 +114,8 @@ static int hmm_pfns_fill(unsigned long addr, unsigned long end,
 		}
 	}
 
+	hmm_vma_handle_migrate_prepare_rollback(hmm_vma_walk, addr, end, &range->hmm_pfns[i]);
+
 	if (migrate && thp_migration_supported() &&
 	    (minfo & MIGRATE_VMA_SELECT_COMPOUND) &&
 	    IS_ALIGNED(addr, HPAGE_PMD_SIZE) &&
@@ -280,6 +287,8 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 		return hmm_record_fault(addr, end, required_fault, walk);
 	}
 
+	hmm_vma_handle_migrate_prepare_rollback(hmm_vma_walk, addr,
+						end, hmm_pfns);
 	pfn = pmd_pfn(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
 		hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS;
@@ -410,6 +419,9 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 
 	new_pfn_flags = pte_pfn(pte) | cpu_flags;
 out:
+	hmm_vma_handle_migrate_prepare_rollback(hmm_vma_walk, addr,
+						addr + PAGE_SIZE,
+						hmm_pfn);
 	*hmm_pfn = (*hmm_pfn & HMM_PFN_INOUT_FLAGS) | new_pfn_flags;
 	return 0;
 
@@ -448,6 +460,9 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 		if (softleaf_is_device_private_write(entry))
 			cpu_flags |= HMM_PFN_WRITE;
 
+		hmm_vma_handle_migrate_prepare_rollback(hmm_vma_walk,
+							start, end,
+							hmm_pfns);
 		/*
 		 * Fully populate the PFN list though subsequent PFNs could be
 		 * inferred, because drivers which are not yet aware of large
@@ -550,6 +565,48 @@ static int migrate_vma_split_folio(struct folio *folio,
 	return 0;
 }
 
+/*
+ * Due to dropping ptl locks for splitting for instance, would we
+ * overwrite already collected pfns? This could happen when pmd
+ * pointing to a page table has vanished and been replaced
+ * with a leaf pmd, or another page table.
+ * In that case unref and unlock the folios,
+ * the pfns of which were collected from the disappeared
+ * page tables.
+ */
+static void hmm_vma_handle_migrate_prepare_rollback(const struct hmm_vma_walk *hmm_vma_walk,
+						    unsigned long start,
+						    unsigned long end,
+						    unsigned long *hmm_pfn)
+{
+	struct hmm_range *range = hmm_vma_walk->range;
+	struct migrate_vma *migrate = range->migrate;
+	struct folio *fault_folio = NULL;
+	enum migrate_vma_info minfo;
+	struct folio *folio;
+	unsigned long i;
+
+	minfo = hmm_select_migrate(range);
+	if (!minfo)
+		return;
+
+	WARN_ON_ONCE(!migrate);
+
+	fault_folio = migrate->fault_page ?
+		page_folio(migrate->fault_page) : NULL;
+
+	for (i = 0; start < end; start += PAGE_SIZE, i++) {
+		if (hmm_pfn_collected(hmm_pfn[i])) {
+			folio = page_folio(hmm_pfn_to_page(hmm_pfn[i]));
+			if (folio != fault_folio)
+				folio_unlock(folio);
+			folio_put(folio);
+			hmm_pfn[i] = hmm_pfn_rollback_collected(hmm_pfn[i]);
+
+		}
+	}
+}
+
 static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
 					      pmd_t *pmdp,
 					      unsigned long start,
@@ -693,6 +750,11 @@ static int hmm_vma_handle_migrate_prepare(const struct mm_walk *walk,
 	pte = ptep_get(ptep);
 
 	if (pte_none(pte)) {
+		hmm_vma_handle_migrate_prepare_rollback(hmm_vma_walk,
+							addr,
+							addr + PAGE_SIZE,
+							hmm_pfn);
+
 		if (vma_is_anonymous(walk->vma)) {
 			*hmm_pfn &= HMM_PFN_INOUT_FLAGS;
 			*hmm_pfn |= HMM_PFN_MIGRATE;
@@ -740,6 +802,10 @@ static int hmm_vma_handle_migrate_prepare(const struct mm_walk *walk,
 		pfn = pte_pfn(pte);
 		if (is_zero_pfn(pfn) &&
 		    (minfo & MIGRATE_VMA_SELECT_SYSTEM)) {
+			hmm_vma_handle_migrate_prepare_rollback(hmm_vma_walk,
+								addr,
+								addr + PAGE_SIZE,
+								hmm_pfn);
 			*hmm_pfn = HMM_PFN_MIGRATE;
 			goto out;
 		}
@@ -916,6 +982,13 @@ static int hmm_vma_walk_split(pmd_t *pmdp,
 	return ret;
 }
 #else
+static void hmm_vma_handle_migrate_prepare_rollback(const struct hmm_vma_walk *hmm_vma_walk,
+						    unsigned long start,
+						    unsigned long end,
+						    unsigned long *hmm_pfn)
+{
+}
+
 static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
 					      pmd_t *pmdp,
 					      unsigned long start,
@@ -1142,6 +1215,9 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 		if (ptep) {
 			lazy_mmu_mode_enable();
 			hmm_vma_walk->ptelocked = true;
+		} else {
+			/* The pte table is gone */
+			hmm_vma_handle_migrate_prepare_rollback(walk->private, addr, end, hmm_pfns);
 		}
 	} else {
 		ptep = pte_offset_map(pmdp, addr);
-- 
2.55.0

[PATCH v14 10/12] mm: enable device page migration from HMM pagewalk

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

HMM pagewalk has now the machinery to do the first phase of
device page migration, collecting the pfns and installing
migration ptes.

Enable migration in hmm_range_fault(), and change migrate_vma_setup()
to use HMM pagewalk path. Two new flags, MIGRATE_VMA_FAULT and
MIGRATE_VMA_WRITE are introduced for migrate_vma_setup(),
to request for faulting missing pages, and requesting write
access.

Also, the migrate_vma_collect*() based paths are now unused,
so delete them.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 include/linux/migrate.h |  10 +-
 mm/hmm.c                |  33 ++-
 mm/migrate_device.c     | 562 ++++------------------------------------
 3 files changed, 83 insertions(+), 522 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 01c5f62a56e9..c73e41625ac2 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -180,6 +180,8 @@ enum migrate_vma_info {
 	MIGRATE_VMA_SELECT_DEVICE_PRIVATE = 1 << 1,
 	MIGRATE_VMA_SELECT_DEVICE_COHERENT = 1 << 2,
 	MIGRATE_VMA_SELECT_COMPOUND = 1 << 3,
+	MIGRATE_VMA_FAULT = 1 << 4,
+	MIGRATE_VMA_WRITE = 1 << 5,
 };
 
 struct migrate_vma {
@@ -217,10 +219,14 @@ struct migrate_vma {
 	struct page		*fault_page;
 };
 
-// TODO: enable migration
 static inline enum migrate_vma_info hmm_select_migrate(struct hmm_range *range)
 {
-	return 0;
+	enum migrate_vma_info minfo;
+
+	minfo = (range->default_flags & HMM_PFN_REQ_MIGRATE) ?
+		range->migrate->flags : 0;
+
+	return minfo;
 }
 
 #ifdef CONFIG_MMU_NOTIFIER
diff --git a/mm/hmm.c b/mm/hmm.c
index e64444cbc62a..87ebd2500df6 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -1562,14 +1562,43 @@ static int hmm_range_fault_locked(struct hmm_range *range, bool *locked)
  *		the invalidation to finish.
  * -EFAULT:     A page was requested to be valid and could not be made valid
  *              ie it has no backing VMA or it is illegal to access
+ * -ERANGE:     The range crosses multiple VMAs, or space for hmm_pfns array
+ *              is too low.
  *
  * This is similar to get_user_pages(), except that it can read the page tables
  * without mutating them (ie causing faults).
  *
  * The mmap lock must be held by the caller and will remain held on return.
  * New users should prefer hmm_range_fault_unlocked_timeout() unless they
- * specifically need to keep the mmap lock held across the call. This helper
- * cannot support VMAs whose fault handlers need to drop the mmap lock.
+ * specifically need to keep the mmap lock held across the call like while
+ * migrating. This helper cannot support VMAs whose fault handlers need to
+ * drop the mmap lock.
+ *
+ * If want to do migration after faulting, call hmm_range_fault() with
+ * range.default_flags of HMM_PFN_REQ_MIGRATE, and optionally
+ * HMM_PFN_REQ_FAULT|HMM_PFN_REQ_WRITE, and initialize range->migrate field.
+ * range->migrate->vma will be populated during the call,
+ * and must be stable across the whole migrate process, which is
+ * why mmap_lock must be held around this call.
+ *
+ * When HMM_PFN_REQ_MIGRATE is set, migration collection may be partial on
+ * return and the caller takes responsibility for completing or aborting it.
+ *
+ * On success, the caller must call migrate_hmm_range_setup() and may then
+ * proceed with the normal migrate_vma sequence: prepare destination pages,
+ * call migrate_vma_pages(), update device mappings as needed, and finally
+ * call migrate_vma_finalize().
+ *
+ * On -EBUSY, the caller may retry hmm_range_fault() using the same range and
+ * PFN array without undoing entries collected by the previous attempt. If
+ * the caller stops retrying, it must abort the partial migration.
+ *
+ * On any other error, or when abandoning a retry, the caller must call
+ * migrate_hmm_range_setup(), migrate_vma_pages() with no valid destination
+ * entries, and migrate_vma_finalize() to abort the partial migration.
+ *
+ * The mmap read lock must remain held until the migration has either been
+ * completed or aborted.
  */
 int hmm_range_fault(struct hmm_range *range)
 {
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 43e571e82cca..1e1a592e1267 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -18,519 +18,6 @@
 #include <asm/tlbflush.h>
 #include "internal.h"
 
-static int migrate_vma_collect_skip(unsigned long start,
-				    unsigned long end,
-				    struct mm_walk *walk)
-{
-	struct migrate_vma *migrate = walk->private;
-	unsigned long addr;
-
-	for (addr = start; addr < end; addr += PAGE_SIZE) {
-		migrate->dst[migrate->npages] = 0;
-		migrate->src[migrate->npages++] = 0;
-	}
-
-	return 0;
-}
-
-static int migrate_vma_collect_hole(unsigned long start,
-				    unsigned long end,
-				    __always_unused int depth,
-				    struct mm_walk *walk)
-{
-	struct migrate_vma *migrate = walk->private;
-	unsigned long addr;
-
-	/* Only allow populating anonymous memory. */
-	if (!vma_is_anonymous(walk->vma))
-		return migrate_vma_collect_skip(start, end, walk);
-
-	if (thp_migration_supported() &&
-		(migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
-		(IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
-		 IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
-		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE |
-						MIGRATE_PFN_COMPOUND;
-		migrate->dst[migrate->npages] = 0;
-		migrate->npages++;
-		migrate->cpages++;
-
-		/*
-		 * Collect the remaining entries as holes, in case we
-		 * need to split later
-		 */
-		return migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
-	}
-
-	for (addr = start; addr < end; addr += PAGE_SIZE) {
-		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
-		migrate->dst[migrate->npages] = 0;
-		migrate->npages++;
-		migrate->cpages++;
-	}
-
-	return 0;
-}
-
-/**
- * migrate_vma_split_folio() - Helper function to split a THP folio
- * @folio: the folio to split
- * @fault_page: struct page associated with the fault if any
- *
- * If @folio is not the folio containing @fault_page, the caller must hold a
- * reference on @folio. The helper consumes that reference.
- *
- * Returns 0 on success
- */
-static int migrate_vma_split_folio(struct folio *folio,
-				   struct page *fault_page)
-{
-	int ret;
-	struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL;
-	struct folio *new_fault_folio = NULL;
-
-	if (folio != fault_folio)
-		folio_lock(folio);
-
-	ret = split_folio(folio);
-	if (ret) {
-		if (folio != fault_folio) {
-			folio_unlock(folio);
-			folio_put(folio);
-		}
-		return ret;
-	}
-
-	new_fault_folio = fault_page ? page_folio(fault_page) : NULL;
-
-	/*
-	 * Ensure the lock is held on the correct
-	 * folio after the split
-	 */
-	if (!new_fault_folio) {
-		folio_unlock(folio);
-		folio_put(folio);
-	} else if (folio != new_fault_folio) {
-		if (new_fault_folio != fault_folio) {
-			folio_get(new_fault_folio);
-			folio_lock(new_fault_folio);
-		}
-		folio_unlock(folio);
-		folio_put(folio);
-	}
-
-	return 0;
-}
-
-/** migrate_vma_collect_huge_pmd - collect THP pages without splitting the
- * folio for device private pages.
- * @pmdp: pointer to pmd entry
- * @start: start address of the range for migration
- * @end: end address of the range for migration
- * @walk: mm_walk callback structure
- * @fault_folio: folio associated with the fault if any
- *
- * Collect the huge pmd entry at @pmdp for migration and set the
- * MIGRATE_PFN_COMPOUND flag in the migrate src entry to indicate that
- * migration will occur at HPAGE_PMD granularity
- */
-static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
-					unsigned long end, struct mm_walk *walk,
-					struct folio *fault_folio)
-{
-	struct mm_struct *mm = walk->mm;
-	struct folio *folio;
-	struct migrate_vma *migrate = walk->private;
-	spinlock_t *ptl;
-	int ret;
-	unsigned long write = 0;
-
-	ptl = pmd_lock(mm, pmdp);
-	if (pmd_none(*pmdp)) {
-		spin_unlock(ptl);
-		return migrate_vma_collect_hole(start, end, -1, walk);
-	}
-
-	if (pmd_trans_huge(*pmdp)) {
-		if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM)) {
-			spin_unlock(ptl);
-			return migrate_vma_collect_skip(start, end, walk);
-		}
-
-		folio = pmd_folio(*pmdp);
-		if (is_huge_zero_folio(folio)) {
-			spin_unlock(ptl);
-			return migrate_vma_collect_hole(start, end, -1, walk);
-		}
-		if (pmd_write(*pmdp))
-			write = MIGRATE_PFN_WRITE;
-	} else if (!pmd_present(*pmdp)) {
-		const softleaf_t entry = softleaf_from_pmd(*pmdp);
-
-		if (!softleaf_is_device_private(entry) ||
-		    !(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_PRIVATE)) {
-			spin_unlock(ptl);
-			return migrate_vma_collect_skip(start, end, walk);
-		}
-
-		folio = softleaf_to_folio(entry);
-		if (folio->pgmap->owner != migrate->pgmap_owner) {
-			spin_unlock(ptl);
-			return migrate_vma_collect_skip(start, end, walk);
-		}
-
-		if (softleaf_is_device_private_write(entry))
-			write = MIGRATE_PFN_WRITE;
-	} else {
-		spin_unlock(ptl);
-		return -EAGAIN;
-	}
-
-	folio_get(folio);
-	if (folio != fault_folio && unlikely(!folio_trylock(folio))) {
-		spin_unlock(ptl);
-		folio_put(folio);
-		return migrate_vma_collect_skip(start, end, walk);
-	}
-
-	if (thp_migration_supported() &&
-		(migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
-		(IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
-		 IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
-
-		struct page_vma_mapped_walk pvmw = {
-			.ptl = ptl,
-			.address = start,
-			.pmd = pmdp,
-			.vma = walk->vma,
-		};
-
-		unsigned long pfn = page_to_pfn(folio_page(folio, 0));
-
-		migrate->src[migrate->npages] = migrate_pfn(pfn) | write
-						| MIGRATE_PFN_MIGRATE
-						| MIGRATE_PFN_COMPOUND;
-		migrate->dst[migrate->npages++] = 0;
-		migrate->cpages++;
-		ret = set_pmd_migration_entry(&pvmw, folio_page(folio, 0));
-		if (ret) {
-			migrate->npages--;
-			migrate->cpages--;
-			migrate->src[migrate->npages] = 0;
-			migrate->dst[migrate->npages] = 0;
-			goto fallback;
-		}
-		migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
-		spin_unlock(ptl);
-		return 0;
-	}
-
-fallback:
-	spin_unlock(ptl);
-	if (!folio_test_large(folio))
-		goto done;
-	ret = split_folio(folio);
-	if (fault_folio != folio)
-		folio_unlock(folio);
-	folio_put(folio);
-	if (ret)
-		return migrate_vma_collect_skip(start, end, walk);
-	if (pmd_none(pmdp_get_lockless(pmdp)))
-		return migrate_vma_collect_hole(start, end, -1, walk);
-
-done:
-	return -ENOENT;
-}
-
-static int migrate_vma_collect_pmd(pmd_t *pmdp,
-				   unsigned long start,
-				   unsigned long end,
-				   struct mm_walk *walk)
-{
-	struct migrate_vma *migrate = walk->private;
-	struct vm_area_struct *vma = walk->vma;
-	struct mm_struct *mm = vma->vm_mm;
-	unsigned long addr = start, unmapped = 0;
-	spinlock_t *ptl;
-	struct folio *fault_folio = migrate->fault_page ?
-		page_folio(migrate->fault_page) : NULL;
-	pte_t *ptep;
-
-again:
-	if (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)) {
-		int ret = migrate_vma_collect_huge_pmd(pmdp, start, end, walk, fault_folio);
-
-		if (ret == -EAGAIN)
-			goto again;
-		if (ret == 0)
-			return 0;
-	}
-
-	ptep = pte_offset_map_lock(mm, pmdp, start, &ptl);
-	if (!ptep)
-		goto again;
-	lazy_mmu_mode_enable();
-	ptep += (addr - start) / PAGE_SIZE;
-
-	for (; addr < end; addr += PAGE_SIZE, ptep++) {
-		struct dev_pagemap *pgmap;
-		unsigned long mpfn = 0, pfn;
-		struct folio *folio;
-		struct page *page;
-		softleaf_t entry;
-		pte_t pte;
-
-		pte = ptep_get(ptep);
-
-		if (pte_none(pte)) {
-			if (vma_is_anonymous(vma)) {
-				mpfn = MIGRATE_PFN_MIGRATE;
-				migrate->cpages++;
-			}
-			goto next;
-		}
-
-		if (!pte_present(pte)) {
-			/*
-			 * Only care about unaddressable device page special
-			 * page table entry. Other special swap entries are not
-			 * migratable, and we ignore regular swapped page.
-			 */
-			entry = softleaf_from_pte(pte);
-			if (!softleaf_is_device_private(entry))
-				goto next;
-
-			page = softleaf_to_page(entry);
-			pgmap = page_pgmap(page);
-			if (!(migrate->flags &
-				MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
-			    pgmap->owner != migrate->pgmap_owner)
-				goto next;
-
-			folio = page_folio(page);
-			if (folio_test_large(folio)) {
-				int ret;
-
-				/* migrate_vma_split_folio() consumes this reference */
-				if (folio != fault_folio)
-					folio_get(folio);
-				lazy_mmu_mode_disable();
-				pte_unmap_unlock(ptep, ptl);
-				ret = migrate_vma_split_folio(folio,
-							  migrate->fault_page);
-
-				if (ret) {
-					if (unmapped)
-						flush_tlb_range(walk->vma, start, end);
-
-					return migrate_vma_collect_skip(addr, end, walk);
-				}
-
-				goto again;
-			}
-
-			mpfn = migrate_pfn(page_to_pfn(page)) |
-					MIGRATE_PFN_MIGRATE;
-			if (softleaf_is_device_private_write(entry))
-				mpfn |= MIGRATE_PFN_WRITE;
-		} else {
-			pfn = pte_pfn(pte);
-			if (is_zero_pfn(pfn) &&
-			    (migrate->flags & MIGRATE_VMA_SELECT_SYSTEM)) {
-				mpfn = MIGRATE_PFN_MIGRATE;
-				migrate->cpages++;
-				goto next;
-			}
-			page = vm_normal_page(migrate->vma, addr, pte);
-			if (page && !is_zone_device_page(page) &&
-			    !(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM)) {
-				goto next;
-			} else if (page && is_device_coherent_page(page)) {
-				pgmap = page_pgmap(page);
-
-				if (!(migrate->flags &
-					MIGRATE_VMA_SELECT_DEVICE_COHERENT) ||
-					pgmap->owner != migrate->pgmap_owner)
-					goto next;
-			}
-			folio = page ? page_folio(page) : NULL;
-			if (folio && folio_test_large(folio)) {
-				int ret;
-
-				/* migrate_vma_split_folio() consumes this reference */
-				if (folio != fault_folio)
-					folio_get(folio);
-				lazy_mmu_mode_disable();
-				pte_unmap_unlock(ptep, ptl);
-				ret = migrate_vma_split_folio(folio,
-							  migrate->fault_page);
-
-				if (ret) {
-					if (unmapped)
-						flush_tlb_range(walk->vma, start, end);
-
-					return migrate_vma_collect_skip(addr, end, walk);
-				}
-
-				goto again;
-			}
-			mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
-			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
-		}
-
-		if (!page || !page->mapping) {
-			mpfn = 0;
-			goto next;
-		}
-
-		/*
-		 * By getting a reference on the folio we pin it and that blocks
-		 * any kind of migration. Side effect is that it "freezes" the
-		 * pte.
-		 *
-		 * We drop this reference after isolating the folio from the lru
-		 * for non device folio (device folio are not on the lru and thus
-		 * can't be dropped from it).
-		 */
-		folio = page_folio(page);
-		folio_get(folio);
-
-		/*
-		 * We rely on folio_trylock() to avoid deadlock between
-		 * concurrent migrations where each is waiting on the others
-		 * folio lock. If we can't immediately lock the folio we fail this
-		 * migration as it is only best effort anyway.
-		 *
-		 * If we can lock the folio it's safe to set up a migration entry
-		 * now. In the common case where the folio is mapped once in a
-		 * single process setting up the migration entry now is an
-		 * optimisation to avoid walking the rmap later with
-		 * try_to_migrate().
-		 */
-		if (fault_folio == folio || folio_trylock(folio)) {
-			bool anon_exclusive;
-			pte_t swp_pte;
-
-			if (pte_present(pte))
-				flush_cache_page(vma, addr, pte_pfn(pte));
-			anon_exclusive = folio_test_anon(folio) &&
-					  PageAnonExclusive(page);
-			if (anon_exclusive) {
-				pte = ptep_clear_flush(vma, addr, ptep);
-
-				if (folio_try_share_anon_rmap_pte(folio, page)) {
-					set_pte_at(mm, addr, ptep, pte);
-					if (fault_folio != folio)
-						folio_unlock(folio);
-					folio_put(folio);
-					mpfn = 0;
-					goto next;
-				}
-			} else {
-				pte = ptep_get_and_clear(mm, addr, ptep);
-			}
-
-			migrate->cpages++;
-
-			/* Set the dirty flag on the folio now the pte is gone. */
-			if (pte_present(pte) && pte_dirty(pte))
-				folio_mark_dirty(folio);
-
-			/* Setup special migration page table entry */
-			if (mpfn & MIGRATE_PFN_WRITE)
-				entry = make_writable_migration_entry(
-							page_to_pfn(page));
-			else if (anon_exclusive)
-				entry = make_readable_exclusive_migration_entry(
-							page_to_pfn(page));
-			else
-				entry = make_readable_migration_entry(
-							page_to_pfn(page));
-			if (pte_present(pte)) {
-				if (pte_young(pte))
-					entry = make_migration_entry_young(entry);
-				if (pte_dirty(pte))
-					entry = make_migration_entry_dirty(entry);
-			}
-			swp_pte = swp_entry_to_pte(entry);
-			if (pte_present(pte)) {
-				if (pte_soft_dirty(pte))
-					swp_pte = pte_swp_mksoft_dirty(swp_pte);
-				if (pte_uffd(pte))
-					swp_pte = pte_swp_mkuffd(swp_pte);
-			} else {
-				if (pte_swp_soft_dirty(pte))
-					swp_pte = pte_swp_mksoft_dirty(swp_pte);
-				if (pte_swp_uffd(pte))
-					swp_pte = pte_swp_mkuffd(swp_pte);
-			}
-			set_pte_at(mm, addr, ptep, swp_pte);
-
-			/*
-			 * This is like regular unmap: we remove the rmap and
-			 * drop the folio refcount. The folio won't be freed, as
-			 * we took a reference just above.
-			 */
-			folio_remove_rmap_pte(folio, page, vma);
-			folio_put(folio);
-
-			if (pte_present(pte))
-				unmapped++;
-		} else {
-			folio_put(folio);
-			mpfn = 0;
-		}
-
-next:
-		migrate->dst[migrate->npages] = 0;
-		migrate->src[migrate->npages++] = mpfn;
-	}
-
-	/* Only flush the TLB if we actually modified any entries */
-	if (unmapped)
-		flush_tlb_range(walk->vma, start, end);
-
-	lazy_mmu_mode_disable();
-	pte_unmap_unlock(ptep - 1, ptl);
-
-	return 0;
-}
-
-static const struct mm_walk_ops migrate_vma_walk_ops = {
-	.pmd_entry		= migrate_vma_collect_pmd,
-	.pte_hole		= migrate_vma_collect_hole,
-	.walk_lock		= PGWALK_RDLOCK,
-};
-
-/*
- * migrate_vma_collect() - collect pages over a range of virtual addresses
- * @migrate: migrate struct containing all migration information
- *
- * This will walk the CPU page table. For each virtual address backed by a
- * valid page, it updates the src array and takes a reference on the page, in
- * order to pin the page until we lock it and unmap it.
- */
-static void migrate_vma_collect(struct migrate_vma *migrate)
-{
-	struct mmu_notifier_range range;
-
-	/*
-	 * Note that the pgmap_owner is passed to the mmu notifier callback so
-	 * that the registered device driver can skip invalidating device
-	 * private page mappings that won't be migrated.
-	 */
-	mmu_notifier_range_init_owner(&range, MMU_NOTIFY_MIGRATE, 0,
-		migrate->vma->vm_mm, migrate->start, migrate->end,
-		migrate->pgmap_owner);
-	mmu_notifier_invalidate_range_start(&range);
-
-	walk_page_range_vma(migrate->vma, migrate->start, migrate->end,
-			&migrate_vma_walk_ops, migrate);
-
-	mmu_notifier_invalidate_range_end(&range);
-	migrate->end = migrate->start + (migrate->npages << PAGE_SHIFT);
-}
-
 /*
  * migrate_vma_check_page() - check if page is pinned or not
  * @page: struct page to check
@@ -677,8 +164,9 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
  * migrate_vma_setup() - prepare to migrate a range of memory
  * @args: contains the vma, start, and pfns arrays for the migration
  *
- * Returns: negative errno on failures, 0 when 0 or more pages were migrated
- * without an error.
+ * Returns: -EINVAL on invalid arguments, 0 otherwise. A 0 return does not
+ * imply anything was collected; on return @args->cpages holds the number of
+ * pages prepared for migration (possibly 0). See the note on faulting below.
  *
  * Prepare to migrate a range of memory virtual address range by collecting all
  * the pages backing each virtual address in the range, saving them inside the
@@ -688,6 +176,20 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
  * corresponding src array entry.  Then restores any pages that are pinned, by
  * remapping and unlocking those pages.
  *
+ * By default migrate_vma_setup() is best-effort: virtual addresses that cannot
+ * be collected are left with an empty src[] entry and the rest of the range
+ * still proceeds.
+ *
+ * The optional flags MIGRATE_VMA_FAULT and MIGRATE_VMA_WRITE in @args->flags
+ * additionally fault in missing pages before collecting them, for read and for
+ * write respectively. Unlike the best-effort default, if such a fault cannot be
+ * satisfied the whole range is aborted: the partial collection is rolled back,
+ * @args->src is cleared, @args->cpages and @args->npages are reset to 0,
+ * and migrate_vma_setup() still returns 0. The fault error is therefore not
+ * observable through this interface. Callers that need to act on fault failures
+ * should instead drive the migration directly via hmm_range_fault() followed by
+ * migrate_hmm_range_setup(), which preserves the error code.
+ *
  * The caller should then allocate destination memory and copy source memory to
  * it for all those entries (ie with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE
  * flag set).  Once these are allocated and copied, the caller must update each
@@ -739,10 +241,20 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
  */
 int migrate_vma_setup(struct migrate_vma *args)
 {
+	int ret;
 	long nr_pages = (args->end - args->start) >> PAGE_SHIFT;
+	struct hmm_range range = {
+		.notifier = NULL,
+		.hmm_pfns = args->src,
+		.dev_private_owner = args->pgmap_owner,
+		.migrate = args,
+		.default_flags = HMM_PFN_REQ_MIGRATE
+	};
 
 	args->start &= PAGE_MASK;
 	args->end &= PAGE_MASK;
+	range.start = args->start;
+	range.end = args->end;
 	if (!args->vma || is_vm_hugetlb_page(args->vma) ||
 	    (args->vma->vm_flags & VM_SPECIAL) || vma_is_dax(args->vma))
 		return -EINVAL;
@@ -764,10 +276,24 @@ int migrate_vma_setup(struct migrate_vma *args)
 	args->cpages = 0;
 	args->npages = 0;
 
-	migrate_vma_collect(args);
+	if (args->flags & MIGRATE_VMA_FAULT)
+		range.default_flags |= HMM_PFN_REQ_FAULT;
 
-	if (args->cpages)
-		migrate_vma_unmap(args);
+	if (args->flags & MIGRATE_VMA_WRITE)
+		range.default_flags |= HMM_PFN_REQ_FAULT | HMM_PFN_REQ_WRITE;
+
+	ret = hmm_range_fault(&range);
+
+	migrate_hmm_range_setup(&range);
+
+	/* Remove migration PTEs */
+	if (ret) {
+		migrate_vma_pages(args);
+		migrate_vma_finalize(args);
+		memset(args->src, 0, sizeof(*args->src) * nr_pages);
+		args->cpages = 0;
+		args->npages = 0;
+	}
 
 	/*
 	 * At this point pages are locked and unmapped, and thus they have
-- 
2.55.0

[PATCH v14 11/12] lib/test_hmm: add a new testcase for the migrate on fault

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

Enhance the hmm test driver (lib/test_hmm) with migrate on fault case.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Marco Pagani <marpagan@redhat.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 lib/test_hmm.c                         | 130 ++++++++++++++++++++++++-
 lib/test_hmm_uapi.h                    |  21 ++--
 tools/testing/selftests/mm/hmm-tests.c |  54 ++++++++++
 3 files changed, 191 insertions(+), 14 deletions(-)

diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index cd88e8177d0a..b39522c05d83 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -36,6 +36,7 @@
 #define DMIRROR_RANGE_FAULT_TIMEOUT	1000
 #define DEVMEM_CHUNK_SIZE		(256 * 1024 * 1024U)
 #define DEVMEM_CHUNKS_RESERVE		16
+#define PFNS_ARRAY_SIZE			64
 
 /*
  * For device_private pages, dpage is just a dummy struct page
@@ -355,6 +356,7 @@ static int dmirror_range_fault(struct dmirror *dmirror,
 	struct mm_struct *mm = dmirror->notifier.mm;
 	unsigned long timeout =
 		jiffies + msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT);
+	bool migrate = range->default_flags & HMM_PFN_REQ_MIGRATE;
 	int ret;
 
 	while (true) {
@@ -364,9 +366,15 @@ static int dmirror_range_fault(struct dmirror *dmirror,
 		}
 
 		range->notifier_seq = mmu_interval_read_begin(range->notifier);
-		mmap_read_lock(mm);
-		ret = hmm_range_fault(range);
-		mmap_read_unlock(mm);
+
+		/* mmap lock held for whole migrate on fault operation */
+		if (!migrate) {
+			mmap_read_lock(mm);
+			ret = hmm_range_fault(range);
+			mmap_read_unlock(mm);
+		} else {
+			ret = hmm_range_fault(range);
+		}
 		if (ret) {
 			if (ret == -EBUSY)
 				continue;
@@ -382,7 +390,9 @@ static int dmirror_range_fault(struct dmirror *dmirror,
 		break;
 	}
 
-	ret = dmirror_do_fault(dmirror, range);
+	/* update device page table after migration */
+	if (!migrate)
+		ret = dmirror_do_fault(dmirror, range);
 
 	mutex_unlock(&dmirror->mutex);
 out:
@@ -1377,6 +1387,114 @@ static int dmirror_migrate_to_device(struct dmirror *dmirror,
 	return ret;
 }
 
+static int do_fault_and_migrate(struct dmirror *dmirror, struct hmm_range *range)
+{
+	struct migrate_vma *migrate = range->migrate;
+	int ret;
+
+	mmap_read_lock(dmirror->notifier.mm);
+
+	/* Fault-in pages for migration */
+	ret = dmirror_range_fault(dmirror, range);
+	/*
+	 * Set this up even on error: hmm_range_fault() may have collected
+	 * part of the range before failing.
+	 */
+	migrate_hmm_range_setup(range);
+	if (ret) {
+		/*
+		 * dst[] entries are empty, so this marks every collected migration
+		 * as failed. finalize then restores the source mappings and drops
+		 * the associated locks/references.
+		 */
+		migrate_vma_pages(migrate);
+		migrate_vma_finalize(migrate);
+		goto out;
+	}
+
+	pr_debug("Migrating from sys mem to device mem\n");
+
+	dmirror_migrate_alloc_and_copy(migrate, dmirror);
+	migrate_vma_pages(migrate);
+	dmirror_migrate_finalize_and_map(migrate, dmirror);
+	migrate_vma_finalize(migrate);
+out:
+	mmap_read_unlock(dmirror->notifier.mm);
+	return ret;
+}
+
+static int dmirror_fault_and_migrate_to_device(struct dmirror *dmirror,
+					       struct hmm_dmirror_cmd *cmd)
+{
+	unsigned long start, size, end, next;
+	unsigned long src_pfns[PFNS_ARRAY_SIZE] = { 0 };
+	unsigned long dst_pfns[PFNS_ARRAY_SIZE] = { 0 };
+	struct migrate_vma migrate = { 0 };
+	struct hmm_range range = { 0 };
+	struct dmirror_bounce bounce;
+	int ret = 0;
+
+	/* Whole range */
+	start = cmd->addr;
+	size = cmd->npages << PAGE_SHIFT;
+	end = start + size;
+
+	if (!mmget_not_zero(dmirror->notifier.mm)) {
+		ret = -EFAULT;
+		goto out;
+	}
+
+	migrate.pgmap_owner = dmirror->mdevice;
+	migrate.src = src_pfns;
+	migrate.dst = dst_pfns;
+	migrate.flags = MIGRATE_VMA_SELECT_SYSTEM;
+
+	range.migrate = &migrate;
+	range.hmm_pfns = src_pfns;
+	range.pfn_flags_mask = 0;
+	range.default_flags = HMM_PFN_REQ_FAULT | HMM_PFN_REQ_MIGRATE;
+	range.dev_private_owner = dmirror->mdevice;
+	range.notifier = &dmirror->notifier;
+
+	for (next = start; next < end; next = range.end) {
+		range.start = next;
+		range.end = min(end, next + (PFNS_ARRAY_SIZE << PAGE_SHIFT));
+
+		pr_debug("Fault and migrate range start:%#lx end:%#lx\n",
+			 range.start, range.end);
+
+		ret = do_fault_and_migrate(dmirror, &range);
+		if (ret)
+			goto out_mmput;
+	}
+
+	/*
+	 * Return the migrated data for verification.
+	 * Only for pages in device zone
+	 */
+	ret = dmirror_bounce_init(&bounce, start, size);
+	if (ret)
+		goto out_mmput;
+
+	mutex_lock(&dmirror->mutex);
+	ret = dmirror_do_read(dmirror, start, end, &bounce);
+	mutex_unlock(&dmirror->mutex);
+	if (ret == 0) {
+		ret = copy_to_user(u64_to_user_ptr(cmd->ptr), bounce.ptr, bounce.size);
+		if (ret)
+			ret = -EFAULT;
+	}
+
+	cmd->cpages = bounce.cpages;
+	dmirror_bounce_fini(&bounce);
+
+
+out_mmput:
+	mmput(dmirror->notifier.mm);
+out:
+	return ret;
+}
+
 static void dmirror_mkentry(struct dmirror *dmirror, struct hmm_range *range,
 			    unsigned char *perm, unsigned long entry)
 {
@@ -1643,6 +1761,10 @@ static long dmirror_fops_unlocked_ioctl(struct file *filp,
 		ret = dmirror_migrate_to_device(dmirror, &cmd);
 		break;
 
+	case HMM_DMIRROR_MIGRATE_ON_FAULT_TO_DEV:
+		ret = dmirror_fault_and_migrate_to_device(dmirror, &cmd);
+		break;
+
 	case HMM_DMIRROR_MIGRATE_TO_SYS:
 		ret = dmirror_migrate_to_system(dmirror, &cmd);
 		break;
diff --git a/lib/test_hmm_uapi.h b/lib/test_hmm_uapi.h
index ea9b0ec404fb..67d267a3b999 100644
--- a/lib/test_hmm_uapi.h
+++ b/lib/test_hmm_uapi.h
@@ -29,16 +29,17 @@ struct hmm_dmirror_cmd {
 };
 
 /* Expose the address space of the calling process through hmm device file */
-#define HMM_DMIRROR_READ		_IOWR('H', 0x00, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_WRITE		_IOWR('H', 0x01, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_MIGRATE_TO_DEV	_IOWR('H', 0x02, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_MIGRATE_TO_SYS	_IOWR('H', 0x03, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_SNAPSHOT		_IOWR('H', 0x04, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_EXCLUSIVE		_IOWR('H', 0x05, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_CHECK_EXCLUSIVE	_IOWR('H', 0x06, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_RELEASE		_IOWR('H', 0x07, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_FLAGS		_IOWR('H', 0x08, struct hmm_dmirror_cmd)
-#define HMM_DMIRROR_READ_UNLOCKED	_IOWR('H', 0x09, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_READ			_IOWR('H', 0x00, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_WRITE			_IOWR('H', 0x01, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_MIGRATE_TO_DEV		_IOWR('H', 0x02, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_MIGRATE_TO_SYS		_IOWR('H', 0x03, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_SNAPSHOT			_IOWR('H', 0x04, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_EXCLUSIVE			_IOWR('H', 0x05, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_CHECK_EXCLUSIVE		_IOWR('H', 0x06, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_RELEASE			_IOWR('H', 0x07, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_FLAGS			_IOWR('H', 0x08, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_READ_UNLOCKED		_IOWR('H', 0x09, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_MIGRATE_ON_FAULT_TO_DEV	_IOWR('H', 0x0a, struct hmm_dmirror_cmd)
 
 #define HMM_DMIRROR_FLAG_FAIL_ALLOC	(1ULL << 0)
 
diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
index e2642eca0d02..a2d1b8f494b6 100644
--- a/tools/testing/selftests/mm/hmm-tests.c
+++ b/tools/testing/selftests/mm/hmm-tests.c
@@ -334,6 +334,13 @@ static int hmm_migrate_sys_to_dev(int fd,
 	return hmm_dmirror_cmd(fd, HMM_DMIRROR_MIGRATE_TO_DEV, buffer, npages);
 }
 
+static int hmm_migrate_on_fault_sys_to_dev(int fd,
+					   struct hmm_buffer *buffer,
+					   unsigned long npages)
+{
+	return hmm_dmirror_cmd(fd, HMM_DMIRROR_MIGRATE_ON_FAULT_TO_DEV, buffer, npages);
+}
+
 static int hmm_migrate_dev_to_sys(int fd,
 				   struct hmm_buffer *buffer,
 				   unsigned long npages)
@@ -938,6 +945,53 @@ TEST_F(hmm, migrate)
 	hmm_buffer_free(buffer);
 }
 
+
+/*
+ * Fault and migrate anonymous memory to device private memory.
+ */
+TEST_F(hmm, migrate_on_fault)
+{
+	struct hmm_buffer *buffer;
+	unsigned long npages;
+	unsigned long size;
+	unsigned long i;
+	int *ptr;
+	int ret;
+
+	npages = ALIGN(HMM_BUFFER_SIZE, self->page_size) >> self->page_shift;
+	ASSERT_NE(npages, 0);
+	size = npages << self->page_shift;
+
+	buffer = malloc(sizeof(*buffer));
+	ASSERT_NE(buffer, NULL);
+
+	buffer->fd = -1;
+	buffer->size = size;
+	buffer->mirror = malloc(size);
+	ASSERT_NE(buffer->mirror, NULL);
+
+	buffer->ptr = mmap(NULL, size,
+			   PROT_READ | PROT_WRITE,
+			   MAP_PRIVATE | MAP_ANONYMOUS,
+			   buffer->fd, 0);
+	ASSERT_NE(buffer->ptr, MAP_FAILED);
+
+	/* Initialize buffer in system memory. */
+	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
+		ptr[i] = i;
+
+	/* Fault and migrate memory to device. */
+	ret = hmm_migrate_on_fault_sys_to_dev(self->fd, buffer, npages);
+	ASSERT_EQ(ret, 0);
+	ASSERT_EQ(buffer->cpages, npages);
+
+	/* Check what the device read. */
+	for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
+		ASSERT_EQ(ptr[i], i);
+
+	hmm_buffer_free(buffer);
+}
+
 /*
  * Migrate private file memory to device private memory.
  */
-- 
2.55.0

[PATCH v14 12/12] Documentation/mm/hmm: document migration through hmm_range_fault()

[mpenttil]

From: Mika Penttilä <mpenttil@redhat.com>

Describe the two new MIGRATE_VMA_FAULT / MIGRATE_VMA_WRITE flags of
migrate_vma_setup(), and add a section on driving the migration
collection phase directly from hmm_range_fault() via HMM_PFN_REQ_MIGRATE
and migrate_hmm_range_setup().

Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
 Documentation/mm/hmm.rst | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/Documentation/mm/hmm.rst b/Documentation/mm/hmm.rst
index fc1b8dc19825..58424e5f5873 100644
--- a/Documentation/mm/hmm.rst
+++ b/Documentation/mm/hmm.rst
@@ -348,6 +348,13 @@ between device driver specific code and shared common code:
    Currently only anonymous private VMA ranges can be migrated to or from
    system memory and device private memory.
 
+   By default only pages already present are collected. Two additional flags
+   ask migrate_vma_setup() to fault in missing pages first:
+
+   * ``MIGRATE_VMA_FAULT`` faults in missing pages with read access.
+   * ``MIGRATE_VMA_WRITE`` faults in missing pages with write access
+     (implies faulting).
+
    One of the first steps migrate_vma_setup() does is to invalidate other
    device's MMUs with the ``mmu_notifier_invalidate_range_start()`` and
    ``mmu_notifier_invalidate_range_end()`` calls around the page table
@@ -427,6 +434,38 @@ between device driver specific code and shared common code:
 
    The lock can now be released.
 
+Migration collection through hmm_range_fault()
+==============================================
+
+The collection phase of migration (steps 1 and 2 above) can also be driven by
+hmm_range_fault() directly, sharing its page table walk. This lets a driver
+fault in and collect a range for migration in one walk, which is useful for
+migrate on fault.
+
+To do so, the driver sets ``HMM_PFN_REQ_MIGRATE`` in ``range->default_flags``
+and points ``range->migrate`` at a ``struct migrate_vma`` it has filled in
+(``flags``, ``src``, ``dst``, ``pgmap_owner``). Usually ``HMM_PFN_REQ_FAULT``
+(and ``HMM_PFN_REQ_WRITE``) is set as well, so missing pages are faulted in
+before being collected. The mmap_read_lock() has to be held for the whole
+migration, since the vma must stay stable.
+
+hmm_range_fault() collects the entries the same way migrate_vma_setup() does,
+taking a folio reference, locking it and installing a migration PTE. Collected
+entries are marked with ``HMM_PFN_VALID | HMM_PFN_MIGRATE`` in
+``range->hmm_pfns``. If the page tables change while locks are dropped the
+partially collected entries are rolled back automatically.
+
+After hmm_range_fault() returns, the driver calls::
+
+    void migrate_hmm_range_setup(struct hmm_range *range);
+
+to translate ``range->hmm_pfns`` into ``migrate->src[]`` (``migrate->dst[]`` is
+zeroed) and initialize ``migrate->cpages`` and ``migrate->npages``. From here on
+the ``range->migrate`` struct is ready for the rest of the flow (steps 3
+onwards), i.e. migrate_vma_pages() and migrate_vma_finalize(). This should be
+called even on error, since hmm_range_fault() may have collected part of the
+range before failing.
+
 Exclusive access memory
 =======================
 
-- 
2.55.0