[PATCH v4 03/15] mm, swap: fix swap cache index error when retrying reclaim

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

The allocator will reclaim cached slots while scanning. Currently, it
will try again if reclaim found a folio that is already removed from
the swap cache due to a race. But the following lookup will be using the
wrong index. It won't cause any OOB issue since the swap cache index is
truncated upon lookup, but it may lead to reclaiming of an irrelevant
folio.

This should not cause a measurable issue, but we should fix it.

Fixes: fae8595505313 ("mm, swap: avoid reclaiming irrelevant swap cache")
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Chris Li <chrisl@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
---
 mm/swapfile.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4b8ab2cb49ca..4baebd8b48f4 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -212,7 +212,7 @@ static bool swap_is_last_map(struct swap_info_struct *si,
 static int __try_to_reclaim_swap(struct swap_info_struct *si,
 				 unsigned long offset, unsigned long flags)
 {
-	swp_entry_t entry = swp_entry(si->type, offset);
+	const swp_entry_t entry = swp_entry(si->type, offset);
 	struct swap_cluster_info *ci;
 	struct folio *folio;
 	int ret, nr_pages;
@@ -240,13 +240,13 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
 	 * 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) {
+	if (offset < swp_offset(folio->swap) ||
+	    offset >= swp_offset(folio->swap) + nr_pages) {
 		folio_unlock(folio);
 		folio_put(folio);
 		goto again;
 	}
-	offset = swp_offset(entry);
+	offset = swp_offset(folio->swap);
 
 	need_reclaim = ((flags & TTRS_ANYWAY) ||
 			((flags & TTRS_UNMAPPED) && !folio_mapped(folio)) ||
-- 
2.51.0