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Wed, 29 Jul 2026 01:42:42 -0700 (PDT) From: Hao Jia To: akpm@linux-foundation.org, tj@kernel.org, hannes@cmpxchg.org, shakeel.butt@linux.dev, mhocko@kernel.org, yosry@kernel.org, mkoutny@suse.com, nphamcs@gmail.com, chengming.zhou@linux.dev, muchun.song@linux.dev, roman.gushchin@linux.dev Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Hao Jia Subject: [PATCH v3 2/2] mm/zswap: Support batch writeback in shrink_memcg() Date: Wed, 29 Jul 2026 16:42:06 +0800 Message-Id: <20260729084206.77793-3-jiahao.kernel@gmail.com> X-Mailer: git-send-email 2.39.2 (Apple Git-143) In-Reply-To: <20260729084206.77793-1-jiahao.kernel@gmail.com> References: <20260729084206.77793-1-jiahao.kernel@gmail.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Hao Jia 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 setting the writeback batch size to SWAP_CLUSTER_MAX for both the shrink_worker() and zswap_store() paths. Test Setup: - Total memory: 32 GB. - 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 Acked-by: Yosry Ahmed Acked-by: Nhat Pham Signed-off-by: Hao Jia --- mm/zswap.c | 45 ++++++++++++++++++++++++++++++++++++++------- 1 file changed, 38 insertions(+), 7 deletions(-) diff --git a/mm/zswap.c b/mm/zswap.c index 48fc7b575e24..82dddfe74ef7 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -1275,9 +1275,25 @@ static struct shrinker *zswap_alloc_shrinker(void) return shrinker; } -static int shrink_memcg(struct mem_cgroup *memcg) +/* + * 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 +1306,29 @@ 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 +1400,7 @@ static void shrink_worker(struct work_struct *w) goto resched; } - ret = shrink_memcg(memcg); + ret = shrink_memcg(memcg, SWAP_CLUSTER_MAX); /* drop the extra reference */ mem_cgroup_put(memcg); @@ -1493,7 +1524,7 @@ 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, SWAP_CLUSTER_MAX)) { mem_cgroup_put(memcg); goto put_objcg; } -- 2.34.1