mm/swap: revert to single-folio writes for synchronous swap devices

Patch series "swap_ops updates", v2.

This series is a follow on to the swap ops series now in mm-unstable.

The first patch reintroduces direct folio writes for synchronous
swap files, the other two remove the double indirect for file
system based swap.


This patch (of 3):

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.

Link: https://lore.kernel.org/20260723054622.3460249-1-hch@lst.de
Link: https://lore.kernel.org/20260723054622.3460249-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Steve French <sfrench@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 10 insertions, 1 deletions

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