The quilt patch titled Subject: memcg: don't wait writeback completion when release memcg has been removed from the -mm tree. Its filename was memcg-dont-wait-writeback-completion-when-release-memcg.patch

This patch was dropped because an updated version will be issued

------------------------------------------------------ From: Julian Sun sunjunchao@bytedance.com Subject: memcg: don't wait writeback completion when release memcg Date: Thu, 28 Aug 2025 04:45:57 +0800

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 millions 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 it.

This allows us to remove wb_wait_for_completion() while preventing the use-after-free issue.

Link: https://lkml.kernel.org/r/20250827204557.90112-1-sunjunchao@bytedance.com Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing") Signed-off-by: Julian Sun sunjunchao@bytedance.com Acked-by: Tejun Heo tj@kernel.org Cc: Michal Hocko mhocko@suse.com Cc: Roman Gushchin roman.gushchin@linux.dev Cc: Johannes Weiner hannes@cmpxchg.org Cc: Shakeel Butt shakeelb@google.com Cc: Muchun Song songmuchun@bytedance.com Cc: Venkat venkat88@linux.ibm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/memcontrol.h | 7 +++- mm/memcontrol.c | 53 +++++++++++++++++++++++++++++------ 2 files changed, 51 insertions(+), 9 deletions(-)

--- a/include/linux/memcontrol.h~memcg-dont-wait-writeback-completion-when-release-memcg +++ a/include/linux/memcontrol.h @@ -157,11 +157,16 @@ struct mem_cgroup_thresholds { */ #define MEMCG_CGWB_FRN_CNT 4

+struct cgwb_frn_wait { + struct wb_completion done; + struct wait_queue_entry wq_entry; +}; + 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 cgwb_frn_wait *wait; /* tracks in-flight foreign writebacks */ };

/* --- a/mm/memcontrol.c~memcg-dont-wait-writeback-completion-when-release-memcg +++ a/mm/memcontrol.c @@ -3457,7 +3457,7 @@ void mem_cgroup_track_foreign_dirty_slow frn->memcg_id == wb->memcg_css->id) break; if (time_before64(frn->at, oldest_at) && - atomic_read(&frn->done.cnt) == 1) { + atomic_read(&frn->wait->done.cnt) == 1) { oldest = i; oldest_at = frn->at; } @@ -3504,12 +3504,12 @@ void mem_cgroup_flush_foreign(struct bdi * already one in flight. */ if (time_after64(frn->at, now - intv) && - atomic_read(&frn->done.cnt) == 1) { + atomic_read(&frn->wait->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->wait->done); } } } @@ -3700,13 +3700,29 @@ static void mem_cgroup_free(struct mem_c __mem_cgroup_free(memcg); }

+#ifdef CONFIG_CGROUP_WRITEBACK +static int memcg_cgwb_waitq_callback_fn(struct wait_queue_entry *wq_entry, unsigned int mode, + int flags, void *key) +{ + struct cgwb_frn_wait *frn_wait = container_of(wq_entry, + struct cgwb_frn_wait, wq_entry); + + if (!atomic_read(&frn_wait->done.cnt)) { + list_del(&wq_entry->entry); + kfree(frn_wait); + } + + return 0; +} +#endif + static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent) { struct memcg_vmstats_percpu *statc; 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); @@ -3761,9 +3777,17 @@ static struct mem_cgroup *mem_cgroup_all 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 = + for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) { + struct memcg_cgwb_frn *frn = &memcg->cgwb_frn[i]; + + frn->wait = kmalloc(sizeof(struct cgwb_frn_wait), GFP_KERNEL); + if (!frn->wait) + goto fail; + + frn->wait->done = __WB_COMPLETION_INIT(&memcg_cgwb_frn_waitq); + init_wait_func(&frn->wait->wq_entry, memcg_cgwb_waitq_callback_fn); + } #endif #ifdef CONFIG_TRANSPARENT_HUGEPAGE spin_lock_init(&memcg->deferred_split_queue.split_queue_lock); @@ -3773,6 +3797,10 @@ static struct mem_cgroup *mem_cgroup_all lru_gen_init_memcg(memcg); return memcg; fail: +#ifdef CONFIG_CGROUP_WRITEBACK + while (i--) + kfree(memcg->cgwb_frn[i].wait); +#endif mem_cgroup_id_remove(memcg); __mem_cgroup_free(memcg); return ERR_PTR(error); @@ -3910,8 +3938,17 @@ static void mem_cgroup_css_free(struct c 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 cgwb_frn_wait *wait = memcg->cgwb_frn[i].wait; + + /* + * Not necessary to wait for wb completion which might cause task hung, + * only used to free resources. See memcg_cgwb_waitq_callback_fn(). + */ + __add_wait_queue_entry_tail(wait->done.waitq, &wait->wq_entry); + if (atomic_dec_and_test(&wait->done.cnt)) + wake_up_all(wait->done.waitq); + } #endif if (cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_nosocket) static_branch_dec(&memcg_sockets_enabled_key); _

Patches currently in -mm which might be from sunjunchao@bytedance.com are