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 <sunjunchao@xxxxxxxxxxxxx>
---
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
