[PATCH v2 0/2] mm/zswap: Fixes and improves the zswap global shrinker

[PATCH v2 0/2] mm/zswap: Fixes and improves the zswap global shrinker

[Hao Jia]

From: Hao Jia <jiahao1@lixiang.com>

This series fixes and improves the zswap global shrinker (shrink_worker()):
Patch 1: Fix missing global shrinker when memory cgroup is disabled.
Patch 2: Extend shrink_memcg() to support batch writeback and update its
         return value semantics, thereby improving the writeback efficiency
         in the shrink_worker() path.

v1->v2:
    - Add a reschedule check to the -ENOENT return path in shrink_memcg() to
      handle the theoretical issue of prolonged heavy concurrent zswap stores.
    - Remove the shrink_memcg() return value changes part, and include a more
       detailed test report in the commit message.


[v1] https://lore.kernel.org/all/20260714081510.16895-1-jiahao.kernel@gmail.com

Hao Jia (2):
  mm/zswap: Fix global shrinker when memory cgroup is disabled
  mm/zswap: Support batch writeback in shrink_memcg()

 mm/zswap.c | 60 ++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 47 insertions(+), 13 deletions(-)

-- 
2.34.1

[PATCH v2 1/2] mm/zswap: Fix global shrinker when memory cgroup is disabled

[Hao Jia]

From: Hao Jia <jiahao1@lixiang.com>

Zswap writeback on hitting the pool limit is broken when memory cgroup
is disabled, because mem_cgroup_iter() always returns NULL. Therefore,
the global shrinker shrink_worker() always takes the !memcg branch.
After MAX_RECLAIM_RETRIES empty walks, the worker simply gives up, so it
fails to write back anything.

Therefore, when memory cgroup is disabled, fall through with the !memcg
branch and shrink the root memcg directly.

With memcg disabled, shrink_memcg() only returns -ENOENT when the root
LRU is empty, which means the total pages are already below thr. In the
absence of heavy concurrent zswap stores, the loop then safely bails out
via the zswap_total_pages() <= thr check; otherwise, it will resume
shrinking the memcg after processing the reschedule check. For any other
return value from shrink_memcg(), the loop is guaranteed to terminate,
either after MAX_RECLAIM_RETRIES failures or once the threshold is met.

Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
Cc: stable@vger.kernel.org
Suggested-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Yosry Ahmed <yosry@kernel.org>
Reported-by: Yosry Ahmed <yosry@kernel.org>
Closes: https://lore.kernel.org/all/CAO9r8zPVzMKFbCixxD-qgtRrkFxWVrHiZZeLc=eyTPKPVQgX4g@mail.gmail.com
Signed-off-by: Hao Jia <jiahao1@lixiang.com>
---
 mm/zswap.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index b5a17ea20237..48fc7b575e24 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1356,11 +1356,12 @@ static void shrink_worker(struct work_struct *w)
 		} while (memcg && !mem_cgroup_tryget_online(memcg));
 		spin_unlock(&zswap_shrink_lock);
 
-		if (!memcg) {
-			/*
-			 * Continue shrinking without incrementing failures if
-			 * we found candidate memcgs in the last tree walk.
-			 */
+		/*
+		 * A NULL memcg ends a full hierarchy pass (except when memcg is
+		 * disabled, where it is always NULL: fall through to the root LRU).
+		 * Count a failure only if the last pass found no candidates.
+		 */
+		if (!memcg && !mem_cgroup_disabled()) {
 			if (!attempts && ++failures == MAX_RECLAIM_RETRIES)
 				break;
 
@@ -1379,7 +1380,7 @@ static void shrink_worker(struct work_struct *w)
 		 * and failures.
 		 */
 		if (ret == -ENOENT)
-			continue;
+			goto resched;
 		++attempts;
 
 		if (ret && ++failures == MAX_RECLAIM_RETRIES)
-- 
2.34.1

[PATCH v2 2/2] mm/zswap: Support batch writeback in shrink_memcg()

[Hao Jia]

From: Hao Jia <jiahao1@lixiang.com>

Currently, shrink_memcg() writes back at most one entry per-node during
its traversal. This makes shrink_worker() inefficient, as it must
repeatedly re-enter shrink_memcg() to make any substantial progress.
Under high memory pressure, this can cause the writeback speed to be
too slow to keep up with refaults, leading to zswap store failures and
forcing pages to skip zswap and go directly to disk, which results in
an LRU inversion.

To address this, extend shrink_memcg() and rewrite its LRU iteration logic
to support batch writeback. Introduce the nr_to_scan parameter to bound how
many pages are scanned per call. This enables batch writeback in the
shrink_worker() path, while maintaining a low scan budget in the
zswap_store() path.

Test Setup:
- Total memory: 32 GB.
- zswap settings: max_pool_percent=1, accept_threshold_percent=50,
  shrinker_enabled=N.

Test Case 1:
Allocate 512MB of anonymous pages and fill them with random data (to avoid
compression), then use cgroup memory.reclaim to force a large amount of
anonymous pages into zswap. At an interval of 2ms, allocate a 4K anonymous
page where the first 4 bytes are random numbers and the rest are zeros, and
then trigger a reclamation of this 4K anonymous page through cgroup
memory.reclaim. When the pool threshold is reached, shrink_memcg() will
be triggered.
The test data after running for 120s is as follows:
                                Baseline       Patched
shrink_worker wakeups              5,363            85
shrink_memcg calls            11,373,201       180,928
written_back pages                40,212        40,236
zswap_store calls                161,190       168,741
   store succeeded (ret=1)       102,743       127,644
   store rejected (ret=0)         58,447        41,097
   store reject rate                ~36%          ~24%
pool_limit_hit delta              55,826        14,062
pswpout                           98,659        81,333
pswpin                                 2             1

Test Case 2:
To consistently force zswap store failures and trigger shrink_worker(),
the following stress-ng command was run for 120 seconds within a cgroup
limited to a memory.max of 1G:
   bash -c 'echo $$ > /sys/fs/cgroup/zswaptest/cgroup.procs ; \
   exec stress-ng --vm 4 --vm-bytes 4G --vm-keep --vm-method rand-set -t \
120s -q'
The test data after running for 120s is as follows:
                                Baseline       Patched
shrink_worker wakeups              5,640           987
shrink_memcg calls             8,481,500     2,504,818
written_back pages                   260       768,576
zswap_store calls              2,742,756     2,301,414
   store succeeded (ret=1)       934,640     1,308,686
   store rejected (ret=0)      1,808,116       992,728
   store reject rate                ~66%          ~43%
pool_limit_hit delta           1,181,310       101,593
pswpout                        1,808,376     1,761,304
pswpin                         4,288,497     3,902,658

Under identical workloads and runtimes, batching the zswap shrinker
exhibits a significant reduction in both shrink_worker wakeups and
shrink_memcg calls. Furthermore, the sharp drop in both pool_limit_hit
and zswap_store rejections demonstrates that batching the zswap shrinker
effectively mitigates zswap_store failures caused by hitting the pool
limit. This significantly prevents pages from bypassing zswap and falling
back directly to disk, thereby reducing LRU inversion.

Suggested-by: Yosry Ahmed <yosry@kernel.org>
Signed-off-by: Hao Jia <jiahao1@lixiang.com>
---
 mm/zswap.c | 47 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 7 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 48fc7b575e24..6a09b9ebfd25 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1275,9 +1275,27 @@ static struct shrinker *zswap_alloc_shrinker(void)
 	return shrinker;
 }
 
-static int shrink_memcg(struct mem_cgroup *memcg)
+#define NR_ZSWAP_WB_BATCH	64UL
+
+/*
+ * Scan up to @nr_to_scan pages across the per-node zswap LRUs of @memcg
+ * and write back the reclaimable ones.
+ *
+ * Since the second-chance algorithm rotates referenced entries to the
+ * LRU tail, the per-node scan is capped at the current LRU length so
+ * each entry is scanned at most once per call. It is up to the caller
+ * to handle retries, deciding whether to scan another memcg to complete
+ * the full iteration, or to rescan the current memcg to drain its zswap
+ * entries.
+ *
+ * Return: 0 if at least one entry was written back, -EAGAIN if entries
+ * were scanned but none could be written back, or -ENOENT if @memcg has
+ * writeback disabled, is a zombie cgroup, or has empty zswap LRUs.
+ */
+static int shrink_memcg(struct mem_cgroup *memcg, unsigned long nr_to_scan)
 {
-	int nid, shrunk = 0, scanned = 0;
+	unsigned long nr_remaining = nr_to_scan;
+	int nid, shrunk = 0;
 
 	if (!mem_cgroup_zswap_writeback_enabled(memcg))
 		return -ENOENT;
@@ -1290,14 +1308,29 @@ static int shrink_memcg(struct mem_cgroup *memcg)
 		return -ENOENT;
 
 	for_each_node_state(nid, N_NORMAL_MEMORY) {
-		unsigned long nr_to_walk = 1;
+		unsigned long nr_to_walk;
 
+		/*
+		 * Cap the scan at per-node LRU length so each entry is scanned
+		 * at most once per call.
+		 */
+		nr_to_walk = min(nr_remaining,
+				 list_lru_count_one(&zswap_list_lru, nid, memcg));
+		if (!nr_to_walk)
+			continue;
+
+		nr_remaining -= nr_to_walk;
 		shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg,
 					    &shrink_memcg_cb, NULL, &nr_to_walk);
-		scanned += 1 - nr_to_walk;
+		/* Return the unused share of the budget to the pool. */
+		nr_remaining += nr_to_walk;
+
+		if (!nr_remaining)
+			break;
 	}
 
-	if (!scanned)
+	/* Nothing was scanned: every LRU under @memcg was empty. */
+	if (nr_remaining == nr_to_scan)
 		return -ENOENT;
 
 	return shrunk ? 0 : -EAGAIN;
@@ -1369,7 +1402,7 @@ static void shrink_worker(struct work_struct *w)
 			goto resched;
 		}
 
-		ret = shrink_memcg(memcg);
+		ret = shrink_memcg(memcg, NR_ZSWAP_WB_BATCH);
 		/* drop the extra reference */
 		mem_cgroup_put(memcg);
 
@@ -1493,7 +1526,7 @@ bool zswap_store(struct folio *folio)
 	objcg = get_obj_cgroup_from_folio(folio);
 	if (objcg && !obj_cgroup_may_zswap(objcg)) {
 		memcg = get_mem_cgroup_from_objcg(objcg);
-		if (shrink_memcg(memcg)) {
+		if (shrink_memcg(memcg, 1)) {
 			mem_cgroup_put(memcg);
 			goto put_objcg;
 		}
-- 
2.34.1

Re: [PATCH v2 2/2] mm/zswap: Support batch writeback in shrink_memcg()

[Yosry Ahmed]

On Fri, Jul 17, 2026 at 04:51:51PM +0800, Hao Jia wrote:
> From: Hao Jia <jiahao1@lixiang.com>
> 
> Currently, shrink_memcg() writes back at most one entry per-node during
> its traversal. This makes shrink_worker() inefficient, as it must
> repeatedly re-enter shrink_memcg() to make any substantial progress.
> Under high memory pressure, this can cause the writeback speed to be
> too slow to keep up with refaults, leading to zswap store failures and
> forcing pages to skip zswap and go directly to disk, which results in
> an LRU inversion.
> 
> To address this, extend shrink_memcg() and rewrite its LRU iteration logic
> to support batch writeback. Introduce the nr_to_scan parameter to bound how
> many pages are scanned per call. This enables batch writeback in the
> shrink_worker() path, while maintaining a low scan budget in the
> zswap_store() path.
> 
> Test Setup:
> - Total memory: 32 GB.
> - zswap settings: max_pool_percent=1, accept_threshold_percent=50,
>   shrinker_enabled=N.
> 
> Test Case 1:
> Allocate 512MB of anonymous pages and fill them with random data (to avoid
> compression), then use cgroup memory.reclaim to force a large amount of
> anonymous pages into zswap. At an interval of 2ms, allocate a 4K anonymous
> page where the first 4 bytes are random numbers and the rest are zeros, and
> then trigger a reclamation of this 4K anonymous page through cgroup
> memory.reclaim. When the pool threshold is reached, shrink_memcg() will
> be triggered.
> The test data after running for 120s is as follows:
>                                 Baseline       Patched
> shrink_worker wakeups              5,363            85
> shrink_memcg calls            11,373,201       180,928
> written_back pages                40,212        40,236
> zswap_store calls                161,190       168,741
>    store succeeded (ret=1)       102,743       127,644
>    store rejected (ret=0)         58,447        41,097
>    store reject rate                ~36%          ~24%
> pool_limit_hit delta              55,826        14,062
> pswpout                           98,659        81,333
> pswpin                                 2             1
> 
> Test Case 2:
> To consistently force zswap store failures and trigger shrink_worker(),
> the following stress-ng command was run for 120 seconds within a cgroup
> limited to a memory.max of 1G:
>    bash -c 'echo $$ > /sys/fs/cgroup/zswaptest/cgroup.procs ; \
>    exec stress-ng --vm 4 --vm-bytes 4G --vm-keep --vm-method rand-set -t \
> 120s -q'
> The test data after running for 120s is as follows:
>                                 Baseline       Patched
> shrink_worker wakeups              5,640           987
> shrink_memcg calls             8,481,500     2,504,818
> written_back pages                   260       768,576
> zswap_store calls              2,742,756     2,301,414
>    store succeeded (ret=1)       934,640     1,308,686
>    store rejected (ret=0)      1,808,116       992,728
>    store reject rate                ~66%          ~43%
> pool_limit_hit delta           1,181,310       101,593
> pswpout                        1,808,376     1,761,304
> pswpin                         4,288,497     3,902,658
> 
> Under identical workloads and runtimes, batching the zswap shrinker
> exhibits a significant reduction in both shrink_worker wakeups and
> shrink_memcg calls. Furthermore, the sharp drop in both pool_limit_hit
> and zswap_store rejections demonstrates that batching the zswap shrinker
> effectively mitigates zswap_store failures caused by hitting the pool
> limit. This significantly prevents pages from bypassing zswap and falling
> back directly to disk, thereby reducing LRU inversion.
> 
> Suggested-by: Yosry Ahmed <yosry@kernel.org>
> Signed-off-by: Hao Jia <jiahao1@lixiang.com>

Acked-by: Yosry Ahmed <yosry@kernel.org>

Re: [PATCH v2 2/2] mm/zswap: Support batch writeback in shrink_memcg()

[Nhat Pham]

On Fri, Jul 17, 2026 at 1:52 AM Hao Jia <jiahao.kernel@gmail.com> wrote:
>
> From: Hao Jia <jiahao1@lixiang.com>
>
> Currently, shrink_memcg() writes back at most one entry per-node during
> its traversal. This makes shrink_worker() inefficient, as it must
> repeatedly re-enter shrink_memcg() to make any substantial progress.
> Under high memory pressure, this can cause the writeback speed to be
> too slow to keep up with refaults, leading to zswap store failures and
> forcing pages to skip zswap and go directly to disk, which results in
> an LRU inversion.
>
> To address this, extend shrink_memcg() and rewrite its LRU iteration logic
> to support batch writeback. Introduce the nr_to_scan parameter to bound how
> many pages are scanned per call. This enables batch writeback in the
> shrink_worker() path, while maintaining a low scan budget in the
> zswap_store() path.
>
> Test Setup:
> - Total memory: 32 GB.
> - zswap settings: max_pool_percent=1, accept_threshold_percent=50,
>   shrinker_enabled=N.
>
> Test Case 1:
> Allocate 512MB of anonymous pages and fill them with random data (to avoid
> compression), then use cgroup memory.reclaim to force a large amount of
> anonymous pages into zswap. At an interval of 2ms, allocate a 4K anonymous
> page where the first 4 bytes are random numbers and the rest are zeros, and
> then trigger a reclamation of this 4K anonymous page through cgroup
> memory.reclaim. When the pool threshold is reached, shrink_memcg() will
> be triggered.
> The test data after running for 120s is as follows:
>                                 Baseline       Patched
> shrink_worker wakeups              5,363            85
> shrink_memcg calls            11,373,201       180,928
> written_back pages                40,212        40,236
> zswap_store calls                161,190       168,741
>    store succeeded (ret=1)       102,743       127,644
>    store rejected (ret=0)         58,447        41,097
>    store reject rate                ~36%          ~24%
> pool_limit_hit delta              55,826        14,062
> pswpout                           98,659        81,333
> pswpin                                 2             1
>
> Test Case 2:
> To consistently force zswap store failures and trigger shrink_worker(),
> the following stress-ng command was run for 120 seconds within a cgroup
> limited to a memory.max of 1G:
>    bash -c 'echo $$ > /sys/fs/cgroup/zswaptest/cgroup.procs ; \
>    exec stress-ng --vm 4 --vm-bytes 4G --vm-keep --vm-method rand-set -t \
> 120s -q'
> The test data after running for 120s is as follows:
>                                 Baseline       Patched
> shrink_worker wakeups              5,640           987
> shrink_memcg calls             8,481,500     2,504,818
> written_back pages                   260       768,576
> zswap_store calls              2,742,756     2,301,414
>    store succeeded (ret=1)       934,640     1,308,686
>    store rejected (ret=0)      1,808,116       992,728
>    store reject rate                ~66%          ~43%
> pool_limit_hit delta           1,181,310       101,593
> pswpout                        1,808,376     1,761,304
> pswpin                         4,288,497     3,902,658
>

Acked-by: Nhat Pham <nphamcs@gmail.com>

Re: [PATCH v2 0/2] mm/zswap: Fixes and improves the zswap global shrinker

[Andrew Morton]

On Fri, 17 Jul 2026 16:51:49 +0800 Hao Jia <jiahao.kernel@gmail.com> wrote:

> This series fixes and improves the zswap global shrinker (shrink_worker()):
> Patch 1: Fix missing global shrinker when memory cgroup is disabled.
> Patch 2: Extend shrink_memcg() to support batch writeback and update its
>          return value semantics, thereby improving the writeback efficiency
>          in the shrink_worker() path.

Thanks.

[1/2] is a cc:stable fix so it isn't really appropriate to combine this
with [2/2] which doesn't fix any bugs.  Because the two patches may
take different paths into mainline, with different timings.  But that's
OK, I can deal with the splitup if needed.

The [1/2] changelog lacks a description of how the flaw impacts users. 
Please describe this fully and maintain that info within the
changelogging.  This info helps -stable maintainers and others
understand why we're proposing a backport and helps myself and others
with timing decisions.

Finally, AI review might have found an issue:
	https://sashiko.dev/#/patchset/20260717085151.22822-1-jiahao.kernel@gmail.com

Re: [PATCH v2 0/2] mm/zswap: Fixes and improves the zswap global shrinker

[Yosry Ahmed]

On Fri, Jul 17, 2026 at 6:18 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 17 Jul 2026 16:51:49 +0800 Hao Jia <jiahao.kernel@gmail.com> wrote:
>
> > This series fixes and improves the zswap global shrinker (shrink_worker()):
> > Patch 1: Fix missing global shrinker when memory cgroup is disabled.
> > Patch 2: Extend shrink_memcg() to support batch writeback and update its
> >          return value semantics, thereby improving the writeback efficiency
> >          in the shrink_worker() path.
>
> Thanks.
>
> [1/2] is a cc:stable fix so it isn't really appropriate to combine this
> with [2/2] which doesn't fix any bugs.  Because the two patches may
> take different paths into mainline, with different timings.  But that's
> OK, I can deal with the splitup if needed.
>
> The [1/2] changelog lacks a description of how the flaw impacts users.
> Please describe this fully and maintain that info within the
> changelogging.  This info helps -stable maintainers and others
> understand why we're proposing a backport and helps myself and others
> with timing decisions.
>
> Finally, AI review might have found an issue:
>         https://sashiko.dev/#/patchset/20260717085151.22822-1-jiahao.kernel@gmail.com

We discussed this one in the previous version, it's a theoretical
scenario that can already happen today.

Re: [PATCH v2 0/2] mm/zswap: Fixes and improves the zswap global shrinker

[Yosry Ahmed]

On Fri, Jul 17, 2026 at 6:18 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 17 Jul 2026 16:51:49 +0800 Hao Jia <jiahao.kernel@gmail.com> wrote:
>
> > This series fixes and improves the zswap global shrinker (shrink_worker()):
> > Patch 1: Fix missing global shrinker when memory cgroup is disabled.
> > Patch 2: Extend shrink_memcg() to support batch writeback and update its
> >          return value semantics, thereby improving the writeback efficiency
> >          in the shrink_worker() path.
>
> Thanks.
>
> [1/2] is a cc:stable fix so it isn't really appropriate to combine this
> with [2/2] which doesn't fix any bugs.  Because the two patches may
> take different paths into mainline, with different timings.  But that's
> OK, I can deal with the splitup if needed.

Thank you!

>
> The [1/2] changelog lacks a description of how the flaw impacts users.
> Please describe this fully and maintain that info within the
> changelogging.  This info helps -stable maintainers and others
> understand why we're proposing a backport and helps myself and others
> with timing decisions.

The first line in the changelog should be sufficient imo: "Zswap
writeback on hitting the pool limit is broken when memory cgroup is
disabled"

Re: [PATCH v2 0/2] mm/zswap: Fixes and improves the zswap global shrinker

[Andrew Morton]

On Fri, 17 Jul 2026 18:28:04 -0700 Yosry Ahmed <yosry@kernel.org> wrote:

> >
> > The [1/2] changelog lacks a description of how the flaw impacts users.
> > Please describe this fully and maintain that info within the
> > changelogging.  This info helps -stable maintainers and others
> > understand why we're proposing a backport and helps myself and others
> > with timing decisions.
> 
> The first line in the changelog should be sufficient imo: "Zswap
> writeback on hitting the pool limit is broken when memory cgroup is
> disabled"

"broken"?  Perhaps this means "fails to occur".

But what is the userspace-visible impact?  IOW, why are we proposing a
backport?