[PATCH v3 4/6] filemap: do not use folio_contains for swap cache folios

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Currently, none of the folio_contains callers should encounter swap
cache folios.

For fs/ callers, swap cache folios are never part of their workflow.

For filemap and truncate, folio_contains is only used for sanity
checks to verify the folio index matches the expected
lookup / invalidation target.

The swap cache does not utilize filemap or truncate helpers in ways
that would trigger these checks, as it mostly implements its own
cache management.

Shmem won't trigger these sanity checks either unless thing went
wrong, as it would directly trigger a BUG because swap cache index are
unrelated and almost never matches shmem index. Shmem have to handle
mixed values of folios, shadows, and swap entries, so it has its own
way of handling the mapping.

While some filemap helpers works for swap cache space, the swap cache
is different from the page cache in many ways. So this particular helper
will unlikely to work in a helpful way for swap cache folios.

So make it explicit here that folio_contains should not be used for
swap cache folios. This helps to avoid misuse, make swap cache less
exposed and remove the folio_index usage here.

Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: David Hildenbrand <david@redhat.com>
---
 include/linux/pagemap.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index af25fb640463..0c9aff5ec77f 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -935,14 +935,14 @@ static inline struct page *folio_file_page(struct folio *folio, pgoff_t index)
  * @folio: The folio.
  * @index: The page index within the file.
  *
- * Context: The caller should have the page locked in order to prevent
- * (eg) shmem from moving the page between the page cache and swap cache
- * and changing its index in the middle of the operation.
+ * Context: The caller should have the folio locked and ensure
+ * e.g., shmem did not move this folio to the swap cache.
  * Return: true or false.
  */
 static inline bool folio_contains(struct folio *folio, pgoff_t index)
 {
-	return index - folio_index(folio) < folio_nr_pages(folio);
+	VM_WARN_ON_FOLIO(folio_test_swapcache(folio), folio);
+	return index - folio->index < folio_nr_pages(folio);
 }
 
 unsigned filemap_get_folios(struct address_space *mapping, pgoff_t *start,
-- 
2.49.0