[PATCH RFC v2 3/6] fs: Add .unaccount_folio callback

[Ackerley Tng <ackerleytng@google.com>]

Add .unaccount_folio callback to allow filesystems to do accounting-related
updates to the inode or struct address_space mapping, when the folio is
about to be removed from the filemap/page_cache.

.free_folio cannot be used since .free_folio cannot assume that struct
address_space mapping still exists.

From the name, .invalidate_folio and .release_folio seem suitable, but
those are meant only to handle freeing of a folio's private
data. .release_folio is also not called in the truncation path.

An alternative would be to add a more general callback and call that from
filemap_remove_folio() and delete_from_page_cache_batch(). .unaccount_folio
was chosen as it is more specific to the how guest_memfd will be using this
callback in later patches. Also, .unaccount_folio only needs a single call
site.

This further refactoring was considered:

if (mapping->a_ops->unaccount_folio &&
    mapping->a_ops->unaccount_folio(folio))
	... do generic page_cache unaccounting ...

but that was abandoned since a hugetlb folio may not have an associated
mapping.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 Documentation/filesystems/vfs.rst | 8 ++++++++
 include/linux/fs.h                | 1 +
 mm/filemap.c                      | 3 +++
 3 files changed, 12 insertions(+)

diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 670ba66b60e49..5ed5c43d5768b 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -809,6 +809,7 @@ cache in your filesystem.  The following members are defined:
 		sector_t (*bmap)(struct address_space *, sector_t);
 		void (*invalidate_folio) (struct folio *, size_t start, size_t len);
 		bool (*release_folio)(struct folio *, gfp_t);
+		void (*unaccount_folio)(struct folio *folio);
 		void (*free_folio)(struct folio *);
 		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
 		int (*migrate_folio)(struct mapping *, struct folio *dst,
@@ -967,6 +968,13 @@ cache in your filesystem.  The following members are defined:
 	its release_folio will need to ensure this.  Possibly it can
 	clear the uptodate flag if it cannot free private data yet.
 
+``unaccount_folio``
+       unaccount_folio is called under inode lock and struct
+       address_space's xa_lock, just before the folio is removed from
+       the page cache in order to allow updating any kind of
+       accounting on the inode or address_space mapping while the
+       address_space mapping still exists.
+
 ``free_folio``
 	free_folio is called once the folio is no longer visible in the
 	page cache in order to allow the cleanup of any private data.
diff --git a/include/linux/fs.h b/include/linux/fs.h
index a01621fa636a6..c71f327032142 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -422,6 +422,7 @@ struct address_space_operations {
 	sector_t (*bmap)(struct address_space *, sector_t);
 	void (*invalidate_folio) (struct folio *, size_t offset, size_t len);
 	bool (*release_folio)(struct folio *, gfp_t);
+	void (*unaccount_folio)(struct folio *folio);
 	void (*free_folio)(struct folio *folio);
 	ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
 	/*
diff --git a/mm/filemap.c b/mm/filemap.c
index ebd75684cb0a7..ff957929e6087 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -176,6 +176,9 @@ static void filemap_unaccount_folio(struct address_space *mapping,
 		}
 	}
 
+	if (unlikely(mapping->a_ops->unaccount_folio))
+		mapping->a_ops->unaccount_folio(folio);
+
 	/* hugetlb folios do not participate in page cache accounting. */
 	if (folio_test_hugetlb(folio))
 		return;

-- 
2.53.0.414.gf7e9f6c205-goog