Re: [patch] mm, oom: prevent additional oom kills before memory is freed

[Michal Hocko <mhocko@kernel.org>]

On Thu 15-06-17 20:32:39, Tetsuo Handa wrote:
> Michal Hocko wrote:
[...]
> > An alternative would be to allow reaping and exit_mmap race. The unmap
> > part should just work I guess. We just have to be careful to not race
> > with free_pgtables and that shouldn't be too hard to implement (e.g.
> > (ab)use mmap_sem for write there). I haven't thought that through
> > completely though so I might miss something of course.
> 
> I think below one is simpler.
[...]
> @@ -556,25 +553,21 @@ static void oom_reap_task(struct task_struct *tsk)
>  	struct mm_struct *mm = tsk->signal->oom_mm;
>  
>  	/* Retry the down_read_trylock(mmap_sem) a few times */
> -	while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task_mm(tsk, mm))
> +	while (__oom_reap_task_mm(tsk, mm), !test_bit(MMF_OOM_SKIP, &mm->flags)
> +	       && attempts++ < MAX_OOM_REAP_RETRIES)
>  		schedule_timeout_idle(HZ/10);
>  
> -	if (attempts <= MAX_OOM_REAP_RETRIES)
> -		goto done;
> -
> -
> -	pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
> -		task_pid_nr(tsk), tsk->comm);
> -	debug_show_all_locks();
> -
> -done:
> -	tsk->oom_reaper_list = NULL;
> -
>  	/*
>  	 * Hide this mm from OOM killer because it has been either reaped or
>  	 * somebody can't call up_write(mmap_sem).
>  	 */
> -	set_bit(MMF_OOM_SKIP, &mm->flags);
> +	if (!test_and_set_bit(MMF_OOM_SKIP, &mm->flags)) {
> +		pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
> +			task_pid_nr(tsk), tsk->comm);
> +		debug_show_all_locks();
> +	}
> +

How does this _solve_ anything? Why would you even retry when you
_know_ that the reference count dropped to zero. It will never
increment. So the above is basically just schedule_timeout_idle(HZ/10) *
MAX_OOM_REAP_RETRIES before we set MMF_OOM_SKIP. This might be enough
for victim to finish the exit_mmap but it is more a hack^Wworkround
than anything else. You could very well do the sleep without any
obfuscation...
-- 
Michal Hocko
SUSE Labs