[PATCH RFC v2 35/50] KVM: guest_memfd: Support converting to shared in PFN mode

[Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org>]

From: Ackerley Tng <ackerleytng@google.com>

When converting memory attributes to shared, guest_memfd iterates
through folios in the page cache to notify architecture code.

In PFN mode, memory is not backed by page cache folios. To notify
architecture code without unintentionally triggering memory allocation
in dynamic providers for unpopulated ranges, introduce the
lookup_pfn_range provider operation. It returns the PFN and number of
pages of contiguous backing memory, or -ENOENT if not allocated.

Add support for PFN mode when converting to shared by querying
lookup_pfn_range and converting contiguous chunks.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 include/linux/guest_memfd.h | 13 ++++++++++++-
 virt/kvm/guest_memfd.c      | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h
index 10c48f41a6d7e..2f1c679dd154c 100644
--- a/include/linux/guest_memfd.h
+++ b/include/linux/guest_memfd.h
@@ -23,7 +23,16 @@ struct mempolicy;
  *           zeroing if necessary. On success, return 0 with the physical frame
  *           number in @pfn and, if @max_order is non-NULL, the maximum page
  *           order of contiguous backing memory containing @pfn. Returns a
- *           negative error code on failure.
+ *           negative error code on failure. The provider must define
+ *           @lookup_pfn_range when @get_pfn is defined.
+ * @lookup_pfn_range: Look up the PFN for a given index. Returns 0 on success
+ *                    with the physical frame number in @pfn and the number of
+ *                    contiguous pages the mapping is valid for in @nr_pages.
+ *                    The provider does not need to ensure that backing memory
+ *                    is present. If backing memory is not available, return
+ *                    -ENOENT. Return a negative error code on other failure.
+ *                    The provider must define @get_pfn when @lookup_pfn_range
+ *                    is defined.
  *
  * Used by filesystems and device drivers that provide memory for guest_memfd.
  */
@@ -35,6 +44,8 @@ struct guest_memfd_provider_operations {
 	void (*invalidate_folio)(void *provider, struct folio *folio);
 	int (*get_pfn)(void *provider, pgoff_t index, unsigned long *pfn,
 		       int *max_order);
+	int (*lookup_pfn_range)(void *provider, pgoff_t index,
+				unsigned long *pfn, size_t *nr_pages);
 };
 
 #endif /* _LINUX_GUEST_MEMFD_H */
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index daf4541c3dec6..734107dad63ee 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -143,6 +143,27 @@ static int gmem_provider_get_pfn(struct gmem_inode *gi, pgoff_t index,
 	return 0;
 }
 
+#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
+static int gmem_provider_lookup_pfn_range(struct gmem_inode *gi, pgoff_t index,
+					  kvm_pfn_t *pfn, size_t *nr_pages)
+{
+	unsigned long raw_pfn;
+	int r;
+
+	r = gi->provider_ops->lookup_pfn_range(gi->provider, index, &raw_pfn,
+					       nr_pages);
+	if (r)
+		return r;
+
+	/* *nr_pages must be > 0 if there is no error. */
+	if (WARN_ON_ONCE(!*nr_pages))
+		return -EINVAL;
+
+	*pfn = raw_pfn;
+	return 0;
+}
+#endif
+
 static void gmem_provider_invalidate_folio(struct gmem_inode *gi,
 					   struct folio *folio)
 {
@@ -783,10 +804,30 @@ static bool kvm_gmem_has_outstanding_references(struct inode *inode,
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
 static void kvm_gmem_make_shared(struct inode *inode, pgoff_t start, pgoff_t end)
 {
+	struct gmem_inode *gi = GMEM_I(inode);
 	struct folio_batch fbatch;
 	pgoff_t next = start;
 	int i;
 
+	if (kvm_gmem_is_pfn_mode(gi)) {
+		pgoff_t index;
+
+		for (index = start; index < end; ) {
+			size_t step = 1;
+			size_t nr_pages;
+			kvm_pfn_t pfn;
+
+			if (!gmem_provider_lookup_pfn_range(gi, index, &pfn, &nr_pages)) {
+				step = min_t(size_t, nr_pages, end - index);
+				kvm_arch_gmem_make_shared(pfn, step);
+			}
+
+			index += step;
+			cond_resched();
+		}
+		return;
+	}
+
 	folio_batch_init(&fbatch);
 	while (filemap_get_folios(inode->i_mapping, &next, end - 1, &fbatch)) {
 		for (i = 0; i < folio_batch_count(&fbatch); ++i) {

-- 
2.56.0.257.gf7497c7d27-goog