Re: [PATCH v3 2/2] mm/zswap: Support batch writeback in shrink_memcg()

[Hao Jia <jiahao.kernel@gmail.com>]

On 2026/7/31 23:17, Johannes Weiner wrote:
> On Fri, Jul 31, 2026 at 03:19:00PM +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,
>> enabling batch writeback for both the shrink_worker() and zswap_store() paths.
>> To prevent shrink unfairness across NUMA nodes caused by a shared global scan
>> quota, limit scanning to up to SWAP_CLUSTER_MAX pages per node and write back
>> any reclaimable entries found.
>>
>> Test Setup:
>> - Total memory: 32 GB, 1 NUMA node.
>> - zswap settings: accept_threshold_percent=50, shrinker_enabled=N.
>>
>> Test Case 1:
>> Set max_pool_percent=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 reclamation of this 4K 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          169
>> shrink_memcg calls            11,373,201      350,703
>> written_back pages                40,212       40,241
>> zswap_store calls                161,190      163,753
>>     store succeeded (ret=1)       102,743      117,183
>>     store rejected (ret=0)         58,447       46,570
>>     store reject rate                ~36%        ~28%
>> pool_limit_hit delta              55,826       33,760
>> pswpout                           98,659       86,811
>> pswpin                                 2            0
>>
>> Test Case 2:
>> We evaluated the following two sub-configurations using stress-ng inside
>> a cgroup capped at memory.max=1G for 120 seconds:
>>    Test Case 2a (max_pool_percent=1): Continuously triggers the global
>>    zswap pool limit, thereby waking up shrink_worker() to perform asynchronous
>>    shrinking.
>>    Test Case 2b (zswap.max=320M, max_pool_percent=50): Continuously triggers
>>    the cgroup's zswap.max limit, thereby invoking synchronous shrinking.
>> Command executed for both setups:
>>     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'
>>
>> Test Case 2a (max_pool_percent=1):
>>                                  Baseline       Patched
>> shrink_worker wakeups              5,640         1,308
>> shrink_memcg calls             8,481,500     3,140,972
>> written_back pages                   260       468,216
>> zswap_store calls              2,742,756     2,011,269
>>     store succeeded (ret=1)       934,640       947,988
>>     store rejected (ret=0)      1,808,116     1,063,281
>>     store reject rate                ~66%          ~52%
>> pool_limit_hit delta           1,181,310       196,882
>> pswpout                        1,808,376     1,531,497
>> pswpin                         4,288,497     3,635,365
>> Test Case 2b (zswap.max=320M, max_pool_percent=50):
>>                                  Baseline       Patched
>> shrink_worker wakeups                 0              0
>> shrink_memcg calls              687,608         54,002
>> written_back pages              639,176        846,663
>> zswap_store calls             1,224,222      1,228,548
>>     store succeeded (ret=1)      992,816      1,208,123
>>     store rejected (ret=0)       231,431         20,425
>>     store reject rate               ~19%            ~2%
>> pool_limit_hit delta                  0              0
>> pswpout                         870,745        867,360
>> pswpin                        1,707,823      1,216,814
>>
>> Under identical workloads and runtimes, batched zswap shrinking
>> exhibits a significant reduction in both shrink_worker() wakeups
>> and shrink_memcg() calls. Furthermore, the sharp drop in both pswpin
>> and zswap_store() rejections demonstrates that batching zswap shrink
>> operations 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>
>> Acked-by: Yosry Ahmed <yosry@kernel.org>
>> Acked-by: Nhat Pham <nphamcs@gmail.com>
>> Signed-off-by: Hao Jia <jiahao1@lixiang.com>
>> ---
>>   mm/zswap.c | 30 ++++++++++++++++++++++++++++--
>>   1 file changed, 28 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 48fc7b575e24..d406c14925d8 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -1275,6 +1275,21 @@ static struct shrinker *zswap_alloc_shrinker(void)
>>   	return shrinker;
>>   }
>>   
>> +/*
>> + * Scan up to SWAP_CLUSTER_MAX pages on each per-node zswap LRU 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)
>>   {
>>   	int nid, shrunk = 0, scanned = 0;
>> @@ -1290,13 +1305,24 @@ 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, node_budget;
>> +
>> +		/*
>> +		 * Cap the scan at the per-node LRU length so each entry is
>> +		 * scanned at most once per call.
>> +		 */
>> +		node_budget = min(SWAP_CLUSTER_MAX,
>> +			     list_lru_count_one(&zswap_list_lru, nid, memcg));
> 
> AFAICS you can just do unsigned long nr_to_walk = SWAP_CLUSTER_MAX.
> 
> __list_lru_walk_one() does a list_for_each_safe() that will exit the
> same way whether you hit !nr_to_walk or run out of items.

Wouldn't it be better to ensure that each entry is scanned at most once 
per call, particularly when list_lru_count_one(nid) < SWAP_CLUSTER_MAX?

On one hand, this avoids scanning the same entry multiple times within a 
single pass. Since the second-chance algorithm rotates referenced 
entries to the tail of the LRU, entries on nodes with a large number of 
zswap entries require at least two shrink_memcg() calls to be written 
back, whereas entries on nodes with fewer entries might get written back 
in a single shrink_memcg() call instead. On the other hand, we also 
avoid spinning repeatedly on entries that fail writeback.

Thanks,
Hao

> 
>> +		if (!node_budget)
>> +			continue;
>>   
>> +		nr_to_walk = node_budget;
>>   		shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg,
>>   					    &shrink_memcg_cb, NULL, &nr_to_walk);
>> -		scanned += 1 - nr_to_walk;
>> +		scanned += node_budget - nr_to_walk;
> 
> scanned += SWAP_CLUSTER_MAX - nr_to_walk;