[PATCH 1/7] mm, swap: avoid reclaiming irrelevant swap cache

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Swap allocator will do swap cache reclaim to recycle HAS_CACHE slots for
allocation. It initiates the reclaim from the offset to be reclaimed and
looks up the corresponding folio. The lookup process is lockless, so it's
possible the folio will be removed from the swap cache and given
a different swap entry before the reclaim locks the folio. If
it happens, the reclaim will end up reclaiming an irrelevant folio, and
return wrong return value.

This shouldn't cause any problem with correctness or stability, but
it is indeed confusing and unexpected, and will increase fragmentation,
decrease performance.

Fix this by checking whether the folio is still pointing to the offset
the allocator want to reclaim before reclaiming it.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/swapfile.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 34baefb000b5..c77ffee4af86 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -210,6 +210,7 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
 	int ret, nr_pages;
 	bool need_reclaim;
 
+again:
 	folio = filemap_get_folio(address_space, swap_cache_index(entry));
 	if (IS_ERR(folio))
 		return 0;
@@ -227,8 +228,16 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
 	if (!folio_trylock(folio))
 		goto out;
 
-	/* offset could point to the middle of a large folio */
+	/*
+	 * Offset could point to the middle of a large folio, or folio
+	 * may no longer point to the expected offset before it's locked.
+	 */
 	entry = folio->swap;
+	if (offset < swp_offset(entry) || offset >= swp_offset(entry) + nr_pages) {
+		folio_unlock(folio);
+		folio_put(folio);
+		goto again;
+	}
 	offset = swp_offset(entry);
 
 	need_reclaim = ((flags & TTRS_ANYWAY) ||
-- 
2.48.1