linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: mhocko@kernel.org
Cc: linux-mm@kvack.org, rientjes@google.com, oleg@redhat.com,
	vdavydov@parallels.com, mgorman@techsingularity.net,
	hughd@google.com, riel@redhat.com, akpm@linux-foundation.org,
	linux-kernel@vger.kernel.org
Subject: Re: mm, oom_reaper: How to handle race with oom_killer_disable() ?
Date: Tue, 21 Jun 2016 20:03:17 +0900	[thread overview]
Message-ID: <201606212003.FFB35429.QtMOJFFFOLSHVO@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <20160621083154.GA30848@dhcp22.suse.cz>

Michal Hocko wrote:
> On Mon 13-06-16 13:19:43, Michal Hocko wrote:
> [...]
> > I am trying to remember why we are disabling oom killer before kernel
> > threads are frozen but not really sure about that right away.
> 
> OK, I guess I remember now. Say that a task would depend on a freezable
> kernel thread to get to do_exit (stuck in wait_event etc...). We would
> simply get stuck in oom_killer_disable for ever. So we need to address
> it a different way.
> 
> One way would be what you are proposing but I guess it would be more
> systematic to never call exit_oom_victim on a remote task.  After [1] we
> have a solid foundation to rely only on MMF_REAPED even when TIF_MEMDIE
> is set. It is more code than your patch so I can see a reason to go with
> yours if the following one seems too large or ugly.
> 
> [1] http://lkml.kernel.org/r/1466426628-15074-1-git-send-email-mhocko@kernel.org
> 
> What do you think about the following?

I'm OK with not clearing TIF_MEMDIE from a remote task. But this patch is racy.

> @@ -567,40 +612,23 @@ static void oom_reap_task(struct task_struct *tsk)
>  	while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task(tsk))
>  		schedule_timeout_idle(HZ/10);
>  
> -	if (attempts > MAX_OOM_REAP_RETRIES) {
> -		struct task_struct *p;
> +	tsk->oom_reaper_list = NULL;
>  
> +	if (attempts > MAX_OOM_REAP_RETRIES) {

attempts > MAX_OOM_REAP_RETRIES would mean that down_read_trylock()
continuously failed. But it does not guarantee that the offending task
shall not call up_write(&mm->mmap_sem) and arrives at mmput() from exit_mm()
(as well as other threads which are blocked at down_read(&mm->mmap_sem) in
exit_mm() by the offending task arrive at mmput() from exit_mm()) when the
OOM reaper was preempted at this point.

Therefore, find_lock_task_mm() in requeue_oom_victim() could return NULL and
the OOM reaper could fail to set MMF_OOM_REAPED (and find_lock_task_mm() in
oom_scan_process_thread() could return NULL and the OOM killer could fail to
select next OOM victim as well) when __mmput() got stuck.

So, from the point of view of correctness, there remains an unhandled race
window as long as you depend on find_lock_task_mm() not returning NULL.
You will again ask "does it really matter/occur", and I can't make progress.

>  		pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
>  				task_pid_nr(tsk), tsk->comm);
>  

  reply	other threads:[~2016-06-21 11:56 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-10 14:23 Tetsuo Handa
2016-06-13 11:19 ` Michal Hocko
2016-06-21  8:31   ` Michal Hocko
2016-06-21 11:03     ` Tetsuo Handa [this message]
2016-06-21 11:46       ` Michal Hocko
2016-06-21 13:27         ` Michal Hocko
2016-06-21 15:32           ` Tetsuo Handa
2016-06-21 17:46             ` Michal Hocko
2016-06-21 21:47               ` Tetsuo Handa
2016-06-22  6:40                 ` Michal Hocko
2016-06-22  6:50                   ` Michal Hocko
2016-06-22 10:57                     ` Tetsuo Handa
2016-06-22 12:08                       ` Michal Hocko
2016-06-22 12:15                         ` Michal Hocko

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=201606212003.FFB35429.QtMOJFFFOLSHVO@I-love.SAKURA.ne.jp \
    --to=penguin-kernel@i-love.sakura.ne.jp \
    --cc=akpm@linux-foundation.org \
    --cc=hughd@google.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mgorman@techsingularity.net \
    --cc=mhocko@kernel.org \
    --cc=oleg@redhat.com \
    --cc=riel@redhat.com \
    --cc=rientjes@google.com \
    --cc=vdavydov@parallels.com \
    --subject='Re: mm, oom_reaper: How to handle race with oom_killer_disable() ?' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
on how to clone and mirror all data and code used for this inbox