[PATCH 06/46] mm/filemap: read mapping->a_ops->free_folio under i_lock

[David Hildenbrand (Arm)]

From: Josef Bacik <josef at toxicpanda.com>

memory_failure() -> ... -> filemap_remove_folio() can read mapping->a_ops
after the inode has been freed. Once the folio is deleted from the
pagecache it no longer pins the mapping, so after i_lock is dropped
e.g., a concurrent final iput() can evict and free the inode before
filemap_free_folio() looks up mapping->a_ops->free_folio.

Most callers hold a reference on the inode, but not all.

Documentation/filesystems/vfs.rst explicitly states that ->free_folio()
must not assume the original address_space mapping still exists.

Fix it by loading ->free_folio while i_lock is still held, similar to
what folio_unmap_invalidate() does.

Inline filemap_free_folio() into its remaining caller such that
delete_from_page_cache_batch() also just loads the callback before
dropping the i_lock.

The bug was found by code inspection and has not been reproduced.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: 59c66c5f8c4f ("mm: factor out page cache page freeing into a separate function")
Cc: stable at vger.kernel.org
Signed-off-by: Josef Bacik <josef at toxicpanda.com>
[ david: get rid of filemap_free_folio() entirely; improve patch description ]
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 mm/filemap.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index b74bc1e5015c6..906b013c11eb3 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -227,18 +227,6 @@ void __filemap_remove_folio(struct folio *folio, void *shadow)
 	page_cache_delete(mapping, folio, shadow);
 }
 
-static void filemap_free_folio(const struct address_space *mapping,
-		struct folio *folio)
-{
-	void (*free_folio)(struct folio *);
-
-	free_folio = mapping->a_ops->free_folio;
-	if (free_folio)
-		free_folio(folio);
-
-	folio_put_refs(folio, folio_nr_pages(folio));
-}
-
 /**
  * filemap_remove_folio - Remove folio from page cache.
  * @folio: The folio.
@@ -250,6 +238,7 @@ static void filemap_free_folio(const struct address_space *mapping,
 void filemap_remove_folio(struct folio *folio)
 {
 	struct address_space *mapping = folio->mapping;
+	void (*free_folio)(struct folio *folio);
 
 	BUG_ON(!folio_test_locked(folio));
 	spin_lock(&mapping->host->i_lock);
@@ -258,9 +247,12 @@ void filemap_remove_folio(struct folio *folio)
 	xa_unlock_irq(&mapping->i_pages);
 	if (mapping_shrinkable(mapping))
 		inode_lru_list_add(mapping->host);
+	free_folio = mapping->a_ops->free_folio;
 	spin_unlock(&mapping->host->i_lock);
 
-	filemap_free_folio(mapping, folio);
+	if (free_folio)
+		free_folio(folio);
+	folio_put_refs(folio, folio_nr_pages(folio));
 }
 
 /*
@@ -320,6 +312,7 @@ static void page_cache_delete_batch(struct address_space *mapping,
 void delete_from_page_cache_batch(struct address_space *mapping,
 				  struct folio_batch *fbatch)
 {
+	void (*free_folio)(struct folio *folio);
 	int i;
 
 	if (!folio_batch_count(fbatch))
@@ -337,10 +330,16 @@ void delete_from_page_cache_batch(struct address_space *mapping,
 	xa_unlock_irq(&mapping->i_pages);
 	if (mapping_shrinkable(mapping))
 		inode_lru_list_add(mapping->host);
+	free_folio = mapping->a_ops->free_folio;
 	spin_unlock(&mapping->host->i_lock);
 
-	for (i = 0; i < folio_batch_count(fbatch); i++)
-		filemap_free_folio(mapping, fbatch->folios[i]);
+	for (i = 0; i < folio_batch_count(fbatch); i++) {
+		struct folio *folio = fbatch->folios[i];
+
+		if (free_folio)
+			free_folio(folio);
+		folio_put_refs(folio, folio_nr_pages(folio));
+	}
 }
 
 int filemap_check_errors(struct address_space *mapping)

-- 
2.43.0