[PATCH v2 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 drop it as soon as writeback completes:
mark it PG_dropbehind so the swap dropbehind path frees it once writeback
ends. Synchronous-IO devices complete writeback in this context, so there
the folio is dropped inline instead.

Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
---
 mm/zswap.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 8163e6c5f76c..e37828e02938 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -992,6 +992,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	struct folio *folio;
 	struct mempolicy *mpol;
 	struct swap_info_struct *si;
+	bool sync;
 	int ret = 0;
 
 	/* try to allocate swap cache folio */
@@ -999,6 +1000,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	if (!si)
 		return -EEXIST;
 
+	sync = data_race(si->flags & SWP_SYNCHRONOUS_IO);
+
 	mpol = get_task_policy(current);
 	folio = __swap_cache_alloc_folio(swpentry, GFP_KERNEL, BIT(0), NULL, mpol,
 					 NO_INTERLEAVE_INDEX);
@@ -1013,7 +1016,6 @@ 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 +1048,17 @@ 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);
+	if (!sync)
+		folio_set_dropbehind(folio);
 
 	/* start writeback */
 	__swap_writepage(folio, NULL);
 
+	if (sync) {
+		swap_dropbehind_free_one_folio(folio);
+		return 0;
+	}
+
 out:
 	if (ret) {
 		swap_cache_del_folio(folio);
-- 
2.53.0-Meta