[PATCH v3 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>
Reviewed-by: Baoquan He <bhe@redhat.com>
---
 mm/swapfile.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index a7f60006c52c..5618cd1c4b03 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