+ mm-zswap-plug-the-writeback-io-of-an-lru-walk.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/zswap: plug the writeback IO of an LRU walk
has been added to the -mm mm-new branch.  Its filename is
     mm-zswap-plug-the-writeback-io-of-an-lru-walk.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-zswap-plug-the-writeback-io-of-an-lru-walk.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Alexandre Ghiti <alex@ghiti.fr>
Subject: mm/zswap: plug the writeback IO of an LRU walk
Date: Wed, 07 Oct 2026 17:34:44 +0200

zswap writeback only appends an entry to the pending bio when its slot
directly follows the previous one, and submits the pending bio
otherwise.

Plug the LRU walk so that the block layer can merge what the batch
cannot.

Kernel build (defconfig, -j4) in a 600M memory.max cgroup with the zswap
shrinker enabled, swapping to an NVMe partition with iocost enabled. It
runs alone, and next to a sibling fio random writer with 10 times its
io.weight so that iocost throttles it. Mean +- stddev, counters from the
build's cgroup:

Alone:

                                unplugged       plugged
  build time (s)                1061 +- 2     1057 +- 2   -0.4%
  write requests (k)            622 +- 34     579 +- 21   -6.8%
  pages per write request    1.30 +- 0.02  1.36 +- 0.05   +4.9%
  iocost debt (s)             14.0 +- 2.5   11.0 +- 1.7   -21.6%
  iocost wait (s)              7.7 +- 1.8    7.1 +- 1.1   -7.9%
  IO full pressure (s)        18.3 +- 1.4   17.5 +- 1.3   -4.5%

Next to the writer:

                                unplugged       plugged
  build time (s)               2188 +- 45    2104 +- 75   -3.8%
  write requests (k)            594 +- 21     567 +- 24   -4.6%
  pages per write request    1.30 +- 0.01  1.35 +- 0.02   +3.6%
  iocost debt (s)               648 +- 22     645 +- 29   -0.5%
  iocost wait (s)               624 +- 23     633 +- 29   +1.5%
  IO full pressure (s)          615 +- 31     550 +- 44   -10.6%

Link: https://lore.kernel.org/20261007153507.50715-3-alex@ghiti.fr
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Yosry Ahmed <yosry@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
---

 mm/zswap.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/mm/zswap.c~mm-zswap-plug-the-writeback-io-of-an-lru-walk
+++ a/mm/zswap.c
@@ -20,6 +20,7 @@
 #include <linux/spinlock.h>
 #include <linux/types.h>
 #include <linux/atomic.h>
+#include <linux/blkdev.h>
 #include <linux/swap_ops.h>
 #include <linux/crypto.h>
 #include <linux/scatterlist.h>
@@ -1283,6 +1284,7 @@ static unsigned long zswap_shrinker_scan
 		.encountered_page_in_swapcache = &encountered_page_in_swapcache,
 	};
 	unsigned long shrink_ret;
+	struct blk_plug plug;
 
 	if (!zswap_shrinker_enabled ||
 			!mem_cgroup_zswap_writeback_enabled(sc->memcg)) {
@@ -1290,10 +1292,12 @@ static unsigned long zswap_shrinker_scan
 		return SHRINK_STOP;
 	}
 
+	blk_start_plug(&plug);
 	shrink_ret = list_lru_shrink_walk(&zswap_list_lru, sc, &shrink_memcg_cb,
 		&ctl);
 
 	swap_write_submit(&ctl.io_ctx);
+	blk_finish_plug(&plug);
 
 	if (encountered_page_in_swapcache)
 		return SHRINK_STOP;
@@ -1401,6 +1405,7 @@ static int shrink_memcg(struct mem_cgrou
 {
 	struct zswap_shrink_ctl ctl = {};
 	int nid, shrunk = 0, scanned = 0;
+	struct blk_plug plug;
 
 	if (!mem_cgroup_zswap_writeback_enabled(memcg))
 		return -ENOENT;
@@ -1412,6 +1417,7 @@ static int shrink_memcg(struct mem_cgrou
 	if (memcg && !mem_cgroup_online(memcg))
 		return -ENOENT;
 
+	blk_start_plug(&plug);
 	for_each_node_state(nid, N_NORMAL_MEMORY) {
 		unsigned long nr_to_walk = SWAP_CLUSTER_MAX;
 
@@ -1421,6 +1427,7 @@ static int shrink_memcg(struct mem_cgrou
 	}
 
 	swap_write_submit(&ctl.io_ctx);
+	blk_finish_plug(&plug);
 
 	/* Nothing was scanned: every LRU under @memcg was empty. */
 	if (!scanned)
_

Patches currently in -mm which might be from alex@ghiti.fr are

mm-zswap-batch-the-writeback-io-of-consecutive-entries.patch
mm-zswap-plug-the-writeback-io-of-an-lru-walk.patch