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Wed, 17 Jun 2026 21:49:27 -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 v4 1/5] mm/zswap: Extend shrink_memcg() writeback capability Date: Thu, 18 Jun 2026 12:48:53 +0800 Message-Id: <20260618044857.69439-2-jiahao.kernel@gmail.com> X-Mailer: git-send-email 2.39.2 (Apple Git-143) In-Reply-To: <20260618044857.69439-1-jiahao.kernel@gmail.com> References: <20260618044857.69439-1-jiahao.kernel@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@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. To address this, extend shrink_memcg() and rewrite its LRU iteration logic to support batch writeback. Introduce the nr_to_writeback parameter to support a writeback budget based on compressed size. This enables batch writeback in the shrink_worker() path, while maintaining a low writeback budget in the zswap_store() path. Additionally, to prepare for future proactive writeback, update the return value semantics of shrink_memcg(): a positive value now represents the actual number of compressed bytes written back, 0 indicates that candidates existed but no writeback succeeded, and a negative value represents an error code. Suggested-by: Yosry Ahmed Signed-off-by: Hao Jia --- mm/zswap.c | 116 ++++++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 97 insertions(+), 19 deletions(-) diff --git a/mm/zswap.c b/mm/zswap.c index 761cd699e0a3..d7d031dee4cd 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -160,6 +160,11 @@ struct zswap_pool { char tfm_name[CRYPTO_MAX_ALG_NAME]; }; +struct zswap_shrink_walk_arg { + unsigned long bytes_written; + bool encountered_page_in_swapcache; +}; + /* Global LRU lists shared by all zswap pools. */ static struct list_lru zswap_list_lru; @@ -1089,8 +1094,9 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o void *arg) { struct zswap_entry *entry = container_of(item, struct zswap_entry, lru); - bool *encountered_page_in_swapcache = (bool *)arg; + struct zswap_shrink_walk_arg *walk_arg = arg; swp_entry_t swpentry; + unsigned int length; enum lru_status ret = LRU_REMOVED_RETRY; int writeback_result; @@ -1135,8 +1141,13 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o * Once the lru lock is dropped, the entry might get freed. The * swpentry is copied to the stack, and entry isn't deref'd again * until the entry is verified to still be alive in the tree. + * + * entry->length is also copied while the lock is held, because + * zswap_writeback_entry() frees the entry on success and we still + * need its compressed size to account for writeback. */ swpentry = entry->swpentry; + length = entry->length; /* * It's safe to drop the lock here because we return either @@ -1155,12 +1166,13 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o * into the warmer region. We should terminate shrinking (if we're in the dynamic * shrinker context). */ - if (writeback_result == -EEXIST && encountered_page_in_swapcache) { + if (writeback_result == -EEXIST) { ret = LRU_STOP; - *encountered_page_in_swapcache = true; + walk_arg->encountered_page_in_swapcache = true; } } else { zswap_written_back_pages++; + walk_arg->bytes_written += length; } return ret; @@ -1169,8 +1181,11 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o static unsigned long zswap_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc) { + struct zswap_shrink_walk_arg walk_arg = { + .bytes_written = 0, + .encountered_page_in_swapcache = false, + }; unsigned long shrink_ret; - bool encountered_page_in_swapcache = false; if (!zswap_shrinker_enabled || !mem_cgroup_zswap_writeback_enabled(sc->memcg)) { @@ -1179,9 +1194,9 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker, } shrink_ret = list_lru_shrink_walk(&zswap_list_lru, sc, &shrink_memcg_cb, - &encountered_page_in_swapcache); + &walk_arg); - if (encountered_page_in_swapcache) + if (walk_arg.encountered_page_in_swapcache) return SHRINK_STOP; return shrink_ret ? shrink_ret : SHRINK_STOP; @@ -1275,10 +1290,32 @@ static struct shrinker *zswap_alloc_shrinker(void) return shrinker; } -static int shrink_memcg(struct mem_cgroup *memcg) -{ - int nid, shrunk = 0, scanned = 0; +/* + * The maximum acceptable scan cost factor for writing back + * PAGE_SIZE bytes of compressed data. + */ +#define ZSWAP_WB_SCAN_FACTOR 16UL +#define NR_ZSWAP_WB_BATCH 64UL +/* + * Iterate over the per-node zswap LRUs of @memcg in batches, writing back + * up to @nr_to_writeback * PAGE_SIZE bytes of compressed data. + * + * Return: The number of bytes written back, or -ENOENT if @memcg has + * writeback disabled, is a zombie cgroup, or has empty zswap LRUs. + */ +static long shrink_memcg(struct mem_cgroup *memcg, + unsigned long nr_to_writeback) +{ + struct zswap_shrink_walk_arg walk_arg = { + .bytes_written = 0, + .encountered_page_in_swapcache = false, + }; + u64 bytes_to_writeback = nr_to_writeback << PAGE_SHIFT; + bool memcg_list_is_empty = true; + int nid; + + /* Memcg with zswap writeback disabled are not candidates. */ if (!mem_cgroup_zswap_writeback_enabled(memcg)) return -ENOENT; @@ -1290,24 +1327,65 @@ 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_scan, nr_scanned = 0; + unsigned long remain; + walk_arg.encountered_page_in_swapcache = false; + /* + * Cap by LRU length: bounds rewalks when referenced + * entries keep rotating to the tail. + */ + nr_to_scan = list_lru_count_one(&zswap_list_lru, nid, memcg); + if (!nr_to_scan) + continue; + memcg_list_is_empty = false; + + /* + * Cap by SCAN_FACTOR * remain budget: bounds scan cost + * to the remaining writeback budget. + */ + remain = DIV_ROUND_UP(bytes_to_writeback - walk_arg.bytes_written, PAGE_SIZE); + nr_to_scan = min(nr_to_scan, + remain * ZSWAP_WB_SCAN_FACTOR); - shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg, - &shrink_memcg_cb, NULL, &nr_to_walk); - scanned += 1 - nr_to_walk; + while (nr_scanned < nr_to_scan) { + unsigned long nr_to_walk = min(NR_ZSWAP_WB_BATCH, + nr_to_scan - nr_scanned); + + /* + * Account for the committed budget rather than the walker's + * actual delta. If the list is emptied concurrently, the + * walker visits nothing and nr_scanned would never advance. + */ + nr_scanned += nr_to_walk; + + list_lru_walk_one(&zswap_list_lru, nid, memcg, + &shrink_memcg_cb, + &walk_arg, + &nr_to_walk); + + if (walk_arg.bytes_written >= bytes_to_writeback) + return walk_arg.bytes_written; + + if (walk_arg.encountered_page_in_swapcache) + break; + + cond_resched(); + } } - if (!scanned) + /* Return -ENOENT if all zswap LRU lists are empty. */ + if (memcg_list_is_empty) return -ENOENT; - return shrunk ? 0 : -EAGAIN; + return walk_arg.bytes_written; } static void shrink_worker(struct work_struct *w) { struct mem_cgroup *memcg; - int ret, failures = 0, attempts = 0; + int failures = 0, attempts = 0; unsigned long thr; + long ret; /* Reclaim down to the accept threshold */ thr = zswap_accept_thr_pages(); @@ -1368,7 +1446,7 @@ 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); @@ -1382,7 +1460,7 @@ static void shrink_worker(struct work_struct *w) continue; ++attempts; - if (ret && ++failures == MAX_RECLAIM_RETRIES) + if (ret <= 0 && ++failures == MAX_RECLAIM_RETRIES) break; resched: cond_resched(); @@ -1492,7 +1570,7 @@ 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) <= 0) { mem_cgroup_put(memcg); goto put_objcg; } -- 2.34.1