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Message-ID: <lsq.1576543535.67878392@decadent.org.uk>
Date: Tue, 17 Dec 2019 00:47:44 +0000
From: Ben Hutchings <ben@...adent.org.uk>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC: akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
"Shakeel Butt" <shakeeb@...gle.com>,
"Michal Koutn" <mkoutny@...e.com>,
"Johannes Weiner" <hannes@...xchg.org>,
"Tejun Heo" <tj@...nel.org>,
"Linus Torvalds" <torvalds@...ux-foundation.org>,
"Roman Gushchin" <guro@...com>, "Michal Hocko" <mhocko@...nel.org>
Subject: [PATCH 3.16 130/136] mm: memcg: switch to css_tryget() in
get_mem_cgroup_from_mm()
3.16.80-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Roman Gushchin <guro@...com>
commit 00d484f354d85845991b40141d40ba9e5eb60faf upstream.
We've encountered a rcu stall in get_mem_cgroup_from_mm():
rcu: INFO: rcu_sched self-detected stall on CPU
rcu: 33-....: (21000 ticks this GP) idle=6c6/1/0x4000000000000002 softirq=35441/35441 fqs=5017
(t=21031 jiffies g=324821 q=95837) NMI backtrace for cpu 33
<...>
RIP: 0010:get_mem_cgroup_from_mm+0x2f/0x90
<...>
__memcg_kmem_charge+0x55/0x140
__alloc_pages_nodemask+0x267/0x320
pipe_write+0x1ad/0x400
new_sync_write+0x127/0x1c0
__kernel_write+0x4f/0xf0
dump_emit+0x91/0xc0
writenote+0xa0/0xc0
elf_core_dump+0x11af/0x1430
do_coredump+0xc65/0xee0
get_signal+0x132/0x7c0
do_signal+0x36/0x640
exit_to_usermode_loop+0x61/0xd0
do_syscall_64+0xd4/0x100
entry_SYSCALL_64_after_hwframe+0x44/0xa9
The problem is caused by an exiting task which is associated with an
offline memcg. We're iterating over and over in the do {} while
(!css_tryget_online()) loop, but obviously the memcg won't become online
and the exiting task won't be migrated to a live memcg.
Let's fix it by switching from css_tryget_online() to css_tryget().
As css_tryget_online() cannot guarantee that the memcg won't go offline,
the check is usually useless, except some rare cases when for example it
determines if something should be presented to a user.
A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
css_tryget() instead of css_tryget_online() in task_get_css()").
Johannes:
: The bug aside, it doesn't matter whether the cgroup is online for the
: callers. It used to matter when offlining needed to evacuate all charges
: from the memcg, and so needed to prevent new ones from showing up, but we
: don't care now.
Link: http://lkml.kernel.org/r/20191106225131.3543616-1-guro@fb.com
Signed-off-by: Roman Gushchin <guro@...com>
Acked-by: Johannes Weiner <hannes@...xchg.org>
Acked-by: Tejun Heo <tj@...nel.org>
Reviewed-by: Shakeel Butt <shakeeb@...gle.com>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Michal Koutn <mkoutny@...e.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
mm/memcontrol.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1073,7 +1073,7 @@ static struct mem_cgroup *get_mem_cgroup
if (unlikely(!memcg))
memcg = root_mem_cgroup;
}
- } while (!css_tryget_online(&memcg->css));
+ } while (!css_tryget(&memcg->css));
rcu_read_unlock();
return memcg;
}
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