Re: WARNING in try_charge

Michal Hocko
2018-08-06T19:46:34Z

On Mon 06-08-18 21:45:53, Michal Hocko wrote:
> [CCing Greg - the email thread starts here
> http://lkml.kernel.org/r/0000000000005e979605729c1564@google.com]

now for real

>
> On Mon 06-08-18 12:12:02, syzbot wrote:
> > Hello,
> >
> > syzbot has tested the proposed patch and the reproducer did not trigger
> > crash:
>
> OK, this is reassuring. Btw Greg has pointed out this potential case
> http://lkml.kernel.org/r/xr93in62jy8k.fsf@gthelen.svl.corp.google.com
> but I simply didn't get what he meant. He was suggesting MMF_OOM_SKIP
> but I didn't get why that matters. I didn't think about a race.
>
> So how about this patch:
> From 74d980f8d066d06ada657ebf9b586dbf5668ed26 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Mon, 6 Aug 2018 21:21:24 +0200
> Subject: [PATCH] memcg, oom: be careful about races when warning about no
> reclaimable task
>
> "memcg, oom: move out_of_memory back to the charge path" has added a
> warning triggered when the oom killer cannot find any eligible task
> and so there is no way to reclaim the oom memcg under its hard limit.
> Further charges for such a memcg are forced and therefore the hard limit
> isolation is weakened.
>
> The current warning is however too eager to trigger even when we are not
> really hitting the above condition. Syzbot and Greg Thelen have noticed
> that we can hit this condition even when there is still oom victim
> pending. E.g. the following race is possible:
>
> memcg has two tasks taskA, taskB.
>
> CPU1 (taskA) CPU2 CPU3 (taskB)
> try_charge
> mem_cgroup_out_of_memory try_charge
> select_bad_process(taskB)
> oom_kill_process oom_reap_task
> # No real memory reaped
> mem_cgroup_out_of_memory
> # set taskB -> MMF_OOM_SKIP
> # retry charge
> mem_cgroup_out_of_memory
> oom_lock oom_lock
> select_bad_process(self)
> oom_kill_process(self)
> oom_unlock
> # no eligible task
>
> In fact syzbot test triggered this situation by placing multiple tasks
> into a memcg with hard limit set to 0. So no task really had any memory
> charged to the memcg
>
> : Memory cgroup stats for /ile0: cache:0KB rss:0KB rss_huge:0KB shmem:0KB mapped_file:0KB dirty:0KB writeback:0KB swap:0KB inactive_anon:0KB active_anon:0KB inactive_file:0KB active_file:0KB unevictable:0KB
> : Tasks state (memory values in pages):
> : [ pid ] uid tgid total_vm rss pgtables_bytes swapents oom_score_adj name
> : [ 6569] 0 6562 9427 1 53248 0 0 syz-executor0
> : [ 6576] 0 6576 9426 0 61440 0 0 syz-executor6
> : [ 6578] 0 6578 9426 534 61440 0 0 syz-executor4
> : [ 6579] 0 6579 9426 0 57344 0 0 syz-executor5
> : [ 6582] 0 6582 9426 0 61440 0 0 syz-executor7
> : [ 6584] 0 6584 9426 0 57344 0 0 syz-executor1
>
> so in principle there is indeed nothing reclaimable in this memcg and
> this looks like a misconfiguration. On the other hand we can clearly
> kill all those tasks so it is a bit early to warn and scare users. Do
> that by checking that the current is the oom victim and bypass the
> warning then. The victim is allowed to force charge and terminate to
> release its temporal charge along the way.
>
> Fixes: "memcg, oom: move out_of_memory back to the charge path"
> Noticed-by: Greg Thelen <gthelen@google.com>
> Reported-and-tested-by: syzbot+bab151e82a4e973fa325@syzkaller.appspotmail.com
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> mm/memcontrol.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4603ad75c9a9..1b6eed1bc404 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1703,7 +1703,8 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int
> return OOM_ASYNC;
> }
>
> - if (mem_cgroup_out_of_memory(memcg, mask, order))
> + if (mem_cgroup_out_of_memory(memcg, mask, order) ||
> + tsk_is_oom_victim(current))
> return OOM_SUCCESS;
>
> WARN(1,"Memory cgroup charge failed because of no reclaimable memory! "
> --
> 2.18.0
>
> --
> Michal Hocko
> SUSE Labs

--
Michal Hocko
SUSE Labs