From: Roman Gushchin guro@fb.com Subject: mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

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 ? unix_stream_sendmsg+0x37d/0x3b0 get_signal+0x132/0x7c0 do_signal+0x36/0x640 ? recalc_sigpending+0x17/0x50 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@fb.com Acked-by: Johannes Weiner hannes@cmpxchg.org Acked-by: Tejun Heo tj@kernel.org Reviewed-by: Shakeel Butt shakeeb@google.com Cc: Michal Hocko mhocko@kernel.org Cc: Michal Koutn mkoutny@suse.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memcontrol.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/memcontrol.c~mm-memcg-switch-to-css_tryget-in-get_mem_cgroup_from_mm +++ a/mm/memcontrol.c @@ -960,7 +960,7 @@ struct mem_cgroup *get_mem_cgroup_from_m if (unlikely(!memcg)) memcg = root_mem_cgroup; } - } while (!css_tryget_online(&memcg->css)); + } while (!css_tryget(&memcg->css)); rcu_read_unlock(); return memcg; } _