[RFC PATCH v2 3/3] mm/vmscan: avoid pointless large folio splits without swap

[Xueyuan Chen <xueyuan.chen21@gmail.com>]

From: "Barry Song (Xiaomi)" <baohua@kernel.org>

When swap is disabled, exhausted, or unavailable due to memcg swap
limits, splitting a large anonymous folio cannot make swapout progress.
The fallback only destroys the large folio and inflates split statistics.

Use -E2BIG from folio_alloc_swap() as the explicit signal that splitting
the folio might allow swapout of smaller pieces. For other allocation
failures, keep the existing activation path and avoid the split.

This preserves the split fallback for fragmented or partially available
swap, while avoiding it when there is no backing space for any part of the
folio.

Reported-by: Nanzhe Zhao <zhaonanzhe@xiaomi.com>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bd1b1aa12581..40340a88f78e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1260,6 +1260,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		 */
 		if (folio_test_anon(folio) && folio_test_swapbacked(folio) &&
 				!folio_test_swapcache(folio)) {
+			int ret;
+
 			if (!(sc->gfp_mask & __GFP_IO))
 				goto keep_locked;
 			if (folio_maybe_dma_pinned(folio))
@@ -1278,10 +1280,11 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 				    split_folio_to_list(folio, folio_list))
 					goto activate_locked;
 			}
-			if (folio_alloc_swap(folio)) {
+			ret = folio_alloc_swap(folio);
+			if (ret) {
 				int __maybe_unused order = folio_order(folio);
 
-				if (!folio_test_large(folio))
+				if (!folio_test_large(folio) || ret != -E2BIG)
 					goto activate_locked_split;
 				/* Fallback to swap normal pages */
 				if (split_folio_to_list(folio, folio_list))
-- 
2.47.3