Re: [PATCH] mm, oom: OOM victims do not need to select next OOM victim unless __GFP_NOFAIL.

[Michal Hocko <mhocko@suse.com>]

On Mon 20-08-18 19:37:45, Tetsuo Handa wrote:
> Commit 696453e66630ad45 ("mm, oom: task_will_free_mem should skip
> oom_reaped tasks") changed to select next OOM victim as soon as
> MMF_OOM_SKIP is set. But since OOM victims can try ALLOC_OOM allocation
> and then give up (if !memcg OOM) or can use forced charge and then retry
> (if memcg OOM), OOM victims do not need to select next OOM victim unless
> they are doing __GFP_NOFAIL allocations.

I do not like this at all. It seems hackish to say the least. And more
importantly...

> This is a quick mitigation because syzbot is hitting WARN(1) caused by
> this race window [1]. More robust fix (e.g. make it possible to reclaim
> more memory before MMF_OOM_SKIP is set, wait for some more after
> MMF_OOM_SKIP is set) is a future work.

.. there is already a patch (by Johannes) for that warning IIRC.

> [1] https://syzkaller.appspot.com/bug?id=ea8c7912757d253537375e981b61749b2da69258
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Reported-and-tested-by: syzbot <syzbot+bab151e82a4e973fa325@syzkaller.appspotmail.com>
> ---
>  mm/oom_kill.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 412f434..421c0f6 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -1031,6 +1031,9 @@ bool out_of_memory(struct oom_control *oc)
>  	unsigned long freed = 0;
>  	enum oom_constraint constraint = CONSTRAINT_NONE;
>  
> +	if (tsk_is_oom_victim(current) && !(oc->gfp_mask & __GFP_NOFAIL))
> +		return true;
> +
>  	if (oom_killer_disabled)
>  		return false;
>  
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs