+ filemap-do-not-use-folio_contains-for-swap-cache-folios.patch added to mm-new branch

+ filemap-do-not-use-folio_contains-for-swap-cache-folios.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: filemap: do not use folio_contains for swap cache folios
has been added to the -mm mm-new branch.  Its filename is
     filemap-do-not-use-folio_contains-for-swap-cache-folios.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/filemap-do-not-use-folio_contains-for-swap-cache-folios.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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patches, and acceptance into mm-new is a notification for others take
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------------------------------------------------------
From: Kairui Song <kasong@tencent.com>
Subject: filemap: do not use folio_contains for swap cache folios
Date: Thu, 1 May 2025 02:10:50 +0800

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.

Link: https://lkml.kernel.org/r/20250430181052.55698-5-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Chao Yu <chao@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Sterba <dsterba@suse.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Joanne Koong <joannelkoong@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Qu Wenruo <wqu@suse.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/pagemap.h |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/include/linux/pagemap.h~filemap-do-not-use-folio_contains-for-swap-cache-folios
+++ a/include/linux/pagemap.h
@@ -935,14 +935,14 @@ static inline struct page *folio_file_pa
  * @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,
_

Patches currently in -mm which might be from kasong@tencent.com are

mm-memory-fix-mapcount-refcount-sanity-check-for-mthp-reuse.patch
mm-swap-fix-false-warning-for-large-allocation-with-thp_swap.patch
fuse-drop-usage-of-folio_index.patch
btrfs-drop-usage-of-folio_index.patch
f2fs-drop-usage-of-folio_index.patch
filemap-do-not-use-folio_contains-for-swap-cache-folios.patch
mm-move-folio_index-to-mm-swaph-and-remove-no-longer-needed-helper.patch
mm-swap-remove-no-longer-used-swap-mapping-helper.patch