Re: WARNING in try_charge

[Michal Hocko]

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

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@xxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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@xxxxxxxx>
> 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@xxxxxxxxxx>
> Reported-and-tested-by: syzbot+bab151e82a4e973fa325@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  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