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

[mpenttil@redhat.com]

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