[PATCH v3 3/3] mm: zswap: drop cold writeback folios via swap dropbehind

[Alexandre Ghiti <alex@ghiti.fr>]

zswap writeback decompresses an entry into a fresh swap cache folio and
writes it back. The folio is cold by construction, yet it is left on the
LRU for reclaim to find and free later, wasting a reclaim scan and keeping
cold memory resident longer than necessary.

Allocate the folio off the LRU and mark it PG_dropbehind so the swap
dropbehind path frees it from the swap cache once writeback completes.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
---
 mm/zswap.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 8163e6c5f76c..a35671e837f8 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1013,7 +1013,6 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	 */
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
-	folio_add_lru(folio);
 
 	/*
 	 * folio is locked, and the swapcache is now secured against
@@ -1046,8 +1045,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	/* folio is up to date */
 	folio_mark_uptodate(folio);
 
-	/* move it to the tail of the inactive list after end_writeback */
-	folio_set_reclaim(folio);
+	folio_set_dropbehind(folio);
 
 	/* start writeback */
 	__swap_writepage(folio, NULL);
-- 
2.53.0-Meta