[PATCH 1/3] mm/swap: revert to single-folio writes for synchronous swap devices

[Christoph Hellwig <hch@lst.de>]

Kairui Song reported that zram benefits from submitting each folio
directly instead of batching up I/O because the classic LRU scanning
benefits from clearing the folio writeback bit in the scan loop.

Accommodate that by kicking off writes for synchronous devices for
each iteration.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Chris Li <chrisl@kernel.org>
---
 mm/page_io.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index e4fa7ffffe8b..c984a4023a65 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -358,7 +358,16 @@ static void swap_add_folio(struct swap_io_ctx *ctx, struct folio *folio, int rw)
 	}
 	bvec_set_folio(&sio->bvecs[sio->nr_bvecs], folio, folio_size(folio), 0);
 	sio->len += folio_size(folio);
-	if (++sio->nr_bvecs == ARRAY_SIZE(sio->bvecs)) {
+
+	/*
+	 * Write out the iocb if we filled it, or if the device is synchronous.
+	 *
+	 * The latter is to work around expectations in the classic LRU code
+	 * which make synchronous clearing of the folio writeback flag in the
+	 * reclaim path beneficial.
+	 */
+	if (++sio->nr_bvecs == ARRAY_SIZE(sio->bvecs) ||
+	    (rw == WRITE && (sis->flags & SWP_SYNCHRONOUS_IO))) {
 		if (rw == WRITE)
 			swap_write_submit(ctx);
 		else
-- 
2.53.0