[PATCH 1/2] mm: vmscan: allow writeout of swapcache folios without anon mappings

[Nhat Pham]

Unfaulted zswap readahead folios can be dirty and in swapcache without
an anonymous mapping. pageout() rejects them before swap_writeout(),
so reclaim cannot write them out while their swap entries remain
referenced.

Allow swapcache folios through the filesystem folio check. They already
have swap entries and can use the existing swap writeout path. Anonymous
folios outside swapcache have no mapping and already return PAGE_KEEP.

Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Link: https://lore.kernel.org/all/sashiko-outbox-181078@xxxxxxxxxx/
Fixes: 84798514db50 ("mm: Remove swap_writepage() and shmem_writepage()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
---
 mm/vmscan.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7db6be53f3a9..4b014e3c07bd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -714,7 +714,8 @@ static pageout_t pageout(struct swap_io_ctx *ctx, struct address_space *mapping,
 	 */
 	if (folio_ref_count(folio) != 1 + folio_nr_pages(folio) || !mapping)
 		return PAGE_KEEP;
-	if (!shmem_mapping(mapping) && !folio_test_anon(folio))
+	/* Anonymous folios outside swapcache returned PAGE_KEEP above. */
+	if (!shmem_mapping(mapping) && !folio_test_swapcache(folio))
 		return PAGE_ACTIVATE;
 	if (!folio_clear_dirty_for_io(folio))
 		return PAGE_CLEAN;