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Wed, 20 Aug 2025 04:19:54 -0700 (PDT) From: Julian Sun To: linux-fsdevel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, axboe@kernel.dk, tj@kernel.org Subject: [PATCH] memcg: Don't wait writeback completion when release memcg. Date: Wed, 20 Aug 2025 19:19:40 +0800 Message-Id: <20250820111940.4105766-4-sunjunchao@bytedance.com> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20250820111940.4105766-1-sunjunchao@bytedance.com> References: <20250820111940.4105766-1-sunjunchao@bytedance.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2D11820007 X-Stat-Signature: nbiyrkajkhu9d5xd5wkc7onf7jd6d493 X-Rspam-User: X-HE-Tag: 1755688795-190226 X-HE-Meta: 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 mRk4h+8q f83Bw01J8selJAIIYPkpxgCF2P4Jkm+J4rxqFiQ0X4VJRk4asAXCXZ61C4sNbfbZ2o5v53gwh3QGLYTqlUdZr755vsLlZbJxF1/1SryqKssz/IO3vfXhvUrACkk469uxsmUQzETxQjvxYSzgb0lVr6tTSjXtalrybcQHfgQ8YMIhCdT43IaRT56npWEhecJkJtAbsWMwcwhPWxMy84rkK+Dmx7i5tHFCJCqXvKePpsGemEdeMXX0culBIhpnNWE106AJrn1/joC3SpcM1GxktNi71OhPZX0bk16E0e9BCGWV9mO/2vC2+x18Ud27WvP6kfka9 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Recently, we encountered the following hung task: INFO: task kworker/4:1:1334558 blocked for more than 1720 seconds. [Wed Jul 30 17:47:45 2025] Workqueue: cgroup_destroy css_free_rwork_fn [Wed Jul 30 17:47:45 2025] Call Trace: [Wed Jul 30 17:47:45 2025] __schedule+0x934/0xe10 [Wed Jul 30 17:47:45 2025] ? complete+0x3b/0x50 [Wed Jul 30 17:47:45 2025] ? _cond_resched+0x15/0x30 [Wed Jul 30 17:47:45 2025] schedule+0x40/0xb0 [Wed Jul 30 17:47:45 2025] wb_wait_for_completion+0x52/0x80 [Wed Jul 30 17:47:45 2025] ? finish_wait+0x80/0x80 [Wed Jul 30 17:47:45 2025] mem_cgroup_css_free+0x22/0x1b0 [Wed Jul 30 17:47:45 2025] css_free_rwork_fn+0x42/0x380 [Wed Jul 30 17:47:45 2025] process_one_work+0x1a2/0x360 [Wed Jul 30 17:47:45 2025] worker_thread+0x30/0x390 [Wed Jul 30 17:47:45 2025] ? create_worker+0x1a0/0x1a0 [Wed Jul 30 17:47:45 2025] kthread+0x110/0x130 [Wed Jul 30 17:47:45 2025] ? __kthread_cancel_work+0x40/0x40 [Wed Jul 30 17:47:45 2025] ret_from_fork+0x1f/0x30 The direct cause is that memcg spends a long time waiting for dirty page writeback of foreign memcgs during release. The root causes are: a. The wb may have multiple writeback tasks, containing hundreds of thousands of dirty pages, as shown below: >>> for work in list_for_each_entry("struct wb_writeback_work", \ wb.work_list.address_of_(), "list"): ... print(work.nr_pages, work.reason, hex(work)) ... 900628 WB_REASON_FOREIGN_FLUSH 0xffff969e8d956b40 1116521 WB_REASON_FOREIGN_FLUSH 0xffff9698332a9540 1275228 WB_REASON_FOREIGN_FLUSH 0xffff969d9b444bc0 1099673 WB_REASON_FOREIGN_FLUSH 0xffff969f0954d6c0 1351522 WB_REASON_FOREIGN_FLUSH 0xffff969e76713340 2567437 WB_REASON_FOREIGN_FLUSH 0xffff9694ae208400 2954033 WB_REASON_FOREIGN_FLUSH 0xffff96a22d62cbc0 3008860 WB_REASON_FOREIGN_FLUSH 0xffff969eee8ce3c0 3337932 WB_REASON_FOREIGN_FLUSH 0xffff9695b45156c0 3348916 WB_REASON_FOREIGN_FLUSH 0xffff96a22c7a4f40 3345363 WB_REASON_FOREIGN_FLUSH 0xffff969e5d872800 3333581 WB_REASON_FOREIGN_FLUSH 0xffff969efd0f4600 3382225 WB_REASON_FOREIGN_FLUSH 0xffff969e770edcc0 3418770 WB_REASON_FOREIGN_FLUSH 0xffff96a252ceea40 3387648 WB_REASON_FOREIGN_FLUSH 0xffff96a3bda86340 3385420 WB_REASON_FOREIGN_FLUSH 0xffff969efc6eb280 3418730 WB_REASON_FOREIGN_FLUSH 0xffff96a348ab1040 3426155 WB_REASON_FOREIGN_FLUSH 0xffff969d90beac00 3397995 WB_REASON_FOREIGN_FLUSH 0xffff96a2d7288800 3293095 WB_REASON_FOREIGN_FLUSH 0xffff969dab423240 3293595 WB_REASON_FOREIGN_FLUSH 0xffff969c765ff400 3199511 WB_REASON_FOREIGN_FLUSH 0xffff969a72d5e680 3085016 WB_REASON_FOREIGN_FLUSH 0xffff969f0455e000 3035712 WB_REASON_FOREIGN_FLUSH 0xffff969d9bbf4b00 b. The writeback might severely throttled by wbt, with a speed possibly less than 100kb/s, leading to a very long writeback time. >>> wb.write_bandwidth (unsigned long)24 >>> wb.write_bandwidth (unsigned long)13 The wb_wait_for_completion() here is probably only used to prevent use-after-free. Therefore, we manage 'done' separately and automatically free this memory in finish_writeback_work(). This allows us to remove wb_wait_for_completion() while preventing the use-after-free issue. Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing") Signed-off-by: Julian Sun --- fs/fs-writeback.c | 1 + include/linux/backing-dev-defs.h | 6 ++++++ include/linux/memcontrol.h | 2 +- mm/memcontrol.c | 31 ++++++++++++++++++++++--------- 4 files changed, 30 insertions(+), 10 deletions(-) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 56faf5c3d064..fac3ef7e95ae 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -1136,6 +1136,7 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, work->reason = reason; work->done = done; work->free_work = 1; + work->free_done = 1; wb_queue_work(wb, work); ret = 0; } else { diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h index 2ad261082bba..bf28dd9a4783 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -208,6 +208,12 @@ struct wb_lock_cookie { unsigned long flags; }; +static inline void wb_completion_init(struct wb_completion *done, struct wait_queue_head *waitq) +{ + atomic_set(&done->cnt, 1); + done->waitq = waitq; +} + #ifdef CONFIG_CGROUP_WRITEBACK /** diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index 785173aa0739..80a6bafbb24a 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -161,7 +161,7 @@ struct memcg_cgwb_frn { u64 bdi_id; /* bdi->id of the foreign inode */ int memcg_id; /* memcg->css.id of foreign inode */ u64 at; /* jiffies_64 at the time of dirtying */ - struct wb_completion done; /* tracks in-flight foreign writebacks */ + struct wb_completion *done; /* tracks in-flight foreign writebacks */ }; /* diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 8dd7fbed5a94..6e6a1ce7589a 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3459,7 +3459,7 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio, frn->memcg_id == wb->memcg_css->id) break; if (time_before64(frn->at, oldest_at) && - atomic_read(&frn->done.cnt) == 1) { + atomic_read(&frn->done->cnt) == 1) { oldest = i; oldest_at = frn->at; } @@ -3506,12 +3506,12 @@ void mem_cgroup_flush_foreign(struct bdi_writeback *wb) * already one in flight. */ if (time_after64(frn->at, now - intv) && - atomic_read(&frn->done.cnt) == 1) { + atomic_read(&frn->done->cnt) == 1) { frn->at = 0; trace_flush_foreign(wb, frn->bdi_id, frn->memcg_id); cgroup_writeback_by_id(frn->bdi_id, frn->memcg_id, WB_REASON_FOREIGN_FLUSH, - &frn->done); + frn->done); } } } @@ -3708,7 +3708,7 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent) struct memcg_vmstats_percpu __percpu *pstatc_pcpu; struct mem_cgroup *memcg; int node, cpu; - int __maybe_unused i; + int __maybe_unused i = 0; long error; memcg = kmem_cache_zalloc(memcg_cachep, GFP_KERNEL); @@ -3763,9 +3763,14 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent) INIT_LIST_HEAD(&memcg->objcg_list); #ifdef CONFIG_CGROUP_WRITEBACK INIT_LIST_HEAD(&memcg->cgwb_list); - for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) - memcg->cgwb_frn[i].done = - __WB_COMPLETION_INIT(&memcg_cgwb_frn_waitq); + for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) { + struct memcg_cgwb_frn *frn = &memcg->cgwb_frn[i]; + + frn->done = kmalloc(sizeof(struct wb_completion), GFP_KERNEL); + if (!frn->done) + goto fail; + wb_completion_init(frn->done, &memcg_cgwb_frn_waitq); + } #endif #ifdef CONFIG_TRANSPARENT_HUGEPAGE spin_lock_init(&memcg->deferred_split_queue.split_queue_lock); @@ -3775,6 +3780,10 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent) lru_gen_init_memcg(memcg); return memcg; fail: +#ifdef CONFIG_CGROUP_WRITEBACK + while (i--) + kfree(memcg->cgwb_frn[i].done); +#endif mem_cgroup_id_remove(memcg); __mem_cgroup_free(memcg); return ERR_PTR(error); @@ -3912,8 +3921,12 @@ static void mem_cgroup_css_free(struct cgroup_subsys_state *css) int __maybe_unused i; #ifdef CONFIG_CGROUP_WRITEBACK - for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) - wb_wait_for_completion(&memcg->cgwb_frn[i].done); + for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) { + struct wb_completion *done = memcg->cgwb_frn[i].done; + + if (atomic_dec_and_test(&done->cnt)) + kfree(done); + } #endif if (cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_nosocket) static_branch_dec(&memcg_sockets_enabled_key); -- 2.20.1