linux-kernel - mm->oom_disable

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Date:	Sat, 30 Jul 2011 17:22:38 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...k.frob.com>,
	Tejun Heo <tj@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Matt Fleming <matt.fleming@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Andrey Vagin <avagin@...nvz.org>,
	Frantisek Hrbata <fhrbata@...hat.com>,
	Ying Han <yinghan@...gle.com>
Subject: mm->oom_disable_count is broken

On 07/30, Oleg Nesterov wrote:
>
> So I'd suggest this hack^Wpatch as a quick fix. I still think this
> code is not right, but the patch tries to keep the current logic.

And this reminds me. mm->oom_disable_count looks absolutely broken.
IIRC, I already complained but nobody replied.

What does this counter mean?

	/* How many tasks sharing this mm are OOM_DISABLE */
	atomic_t oom_disable_count;

tasks? processes or threads?

Lets look at oom_adjust_write(),

		if (task->signal->oom_adj == OOM_DISABLE)
			atomic_inc(&task->mm->oom_disable_count);

OK, so it is per-process. No matter how many threads this process
has, mm->oom_disable_count becomes 1 (ignoring CLONE_VM).


However, exit_mm() does:

	if (tsk->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
		atomic_dec(&mm->oom_disable_count);

but this is per-thread! it becomes zero and then negative after
pthread_exit().


copy_process()->copy_mm() seems to think it is per-thread too. But,
bad_fork_cleanup_mm:

	if (p->mm) {
		task_lock(p);
		if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
			atomic_dec(&p->mm->oom_disable_count);
		task_unlock(p);
		mmput(p->mm);
	}

Why do we take task_lock() ? OK, oom_score_adj_write() does task_lock()
too, but this can't help in the multithreaded case? Why copy_mm() checks
->oom_score_adj lockless?

Oleg.

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