[PATCH v4 3/3] mm: zswap: drop cold writeback folios via swap dropbehind

[Alexandre Ghiti]

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@xxxxxxxxxxx>
Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
---
 mm/zswap.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 8163e6c5f76c..d16822a516e8 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
@@ -1046,12 +1045,26 @@ 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);
+
+	/*
+	 * Drop our reference before starting writeback so the swap cache holds
+	 * the only one: the drop in folio_end_writeback() needs that for
+	 * remove_mapping_reclaim() to succeed, otherwise the folio is handed
+	 * back to reclaim instead.
+	 *
+	 * Nothing can free the folio in the meantime: we hold the folio lock
+	 * until writeback starts, PG_writeback then blocks swap cache removal,
+	 * and folio_end_writeback() takes its own reference before clearing
+	 * PG_writeback and donates it to the drop.
+	 */
+	folio_put(folio);
 
 	/* start writeback */
 	__swap_writepage(folio, NULL);
 
+	return 0;
+
 out:
 	if (ret) {
 		swap_cache_del_folio(folio);
-- 
2.53.0-Meta