linux-kernel - Re: [patch] oom: give current access to memory reserves if it has been killed

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Date:	Wed, 31 Mar 2010 19:58:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	anfei <anfei.zhou@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	nishimura@....nes.nec.co.jp,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] oom: give current access to memory reserves if it has
	been killed

On 03/30, David Rientjes wrote:
>
> On Tue, 30 Mar 2010, Oleg Nesterov wrote:
>
> > Note that __oom_kill_task() does force_sig(SIGKILL) which assumes that
> > ->sighand != NULL. This is not true if out_of_memory() is called after
> > current has already passed exit_notify().
>
> We have an even bigger problem if current is in the oom killer at
> exit_notify() since it has already detached its ->mm in exit_mm() :)

Can't understand... I thought that in theory even kmalloc(1) can trigger
oom.

Say, right after exit_mm() we are doing acct_process(), and f_op->write()
needs a page. So, you are saying that in this case __page_cache_alloc()
can never trigger out_of_memory() ?

> > IOW, unless I missed something, it is very easy to hide the process
> > from oom-kill:
> >
> > 	int main()
> > 	{
> > 		pthread_create(memory_hog_func);
> > 		syscall(__NR_exit);
> > 	}
> >
>
> The check for !p->mm was moved in the -mm tree (and the oom killer was
> entirely rewritten in that tree, so I encourage you to work off of it
> instead

OK, but I guess this !p->mm check is still wrong for the same reason.
In fact I do not understand why it is needed in select_bad_process()
right before oom_badness() which checks ->mm too (and this check is
equally wrong).

> with
> oom-avoid-race-for-oom-killed-tasks-detaching-mm-prior-to-exit.patch to
> even after the check for PF_EXITING.  This is set in the exit path before
> the ->mm is detached

Yes. Then I do not understand "if (!p->mm)" completely.

> so if the oom killer finds an already exiting task,
> it will become a no-op since it should eventually free memory and avoids a
> needless oom kill.

No, afaics, And this reminds that I already complained about this
PF_EXITING check.

Once again, p is the group leader. It can be dead (no ->mm, PF_EXITING
is set) but it can have sub-threads. This means, unless I missed something,
any user can trivially disable select_bad_process() forever.

Well. Looks like, -mm has a lot of changes in oom_kill.c. Perhaps it
would be better to fix these mt bugs first...

Say, oom_forkbomb_penalty() does list_for_each_entry(tsk->children).
Again, this is not right even if we forget about !child->mm check.
This list_for_each_entry() can only see the processes forked by the
main thread.

Likewise, oom_kill_process()->list_for_each_entry() is not right too.

Hmm. Why oom_forkbomb_penalty() does thread_group_cputime() under
task_lock() ? It seems, ->alloc_lock() is only needed for get_mm_rss().

Oleg.

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