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

[Yosry Ahmed <yosry@kernel.org>]

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>