linux-kernel - Re: [PATCH -mm] proc: don't take ->siglock for /proc/pid/oom

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Date:	Wed, 31 Mar 2010 20:59:50 +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 -mm] proc: don't take ->siglock for /proc/pid/oom_adj

On 03/30, David Rientjes wrote:
>
> On Tue, 30 Mar 2010, Oleg Nesterov wrote:
>
> > ->siglock is no longer needed to access task->signal, change
> > oom_adjust_read() and oom_adjust_write() to read/write oom_adj
> > lockless.
> >
> > Yes, this means that "echo 2 >oom_adj" and "echo 1 >oom_adj"
> > can race and the second write can win, but I hope this is OK.
>
> Ok, but could you base this on -mm at
> http://userweb.kernel.org/~akpm/mmotm/ since an additional tunable has
> been added (oom_score_adj), which does the same thing?

David, I just can't understand why
	oom-badness-heuristic-rewrite.patch
duplicates the related code in fs/proc/base.c and why it preserves
the deprecated signal->oom_adj.

OK. Please forget about lock_task_sighand/signal issues. Can't we kill
signal->oom_adj and create a single helper for both
/proc/pid/{oom_adj,oom_score_adj} ?

	static ssize_t oom_any_adj_write(struct file *file, const char __user *buf,
						size_t count, bool deprecated_mode)
	{
		struct task_struct *task;
		char buffer[PROC_NUMBUF];
		unsigned long flags;
		long oom_score_adj;
		int err;

		memset(buffer, 0, sizeof(buffer));
		if (count > sizeof(buffer) - 1)
			count = sizeof(buffer) - 1;
		if (copy_from_user(buffer, buf, count))
			return -EFAULT;

		err = strict_strtol(strstrip(buffer), 0, &oom_score_adj);
		if (err)
			return -EINVAL;

		if (depraceted_mode) {
			 if (oom_score_adj == OOM_ADJUST_MAX)
				oom_score_adj = OOM_SCORE_ADJ_MAX;
			 else
				oom_score_adj = (oom_score_adj * OOM_SCORE_ADJ_MAX) /
						-OOM_DISABLE;
		}

		if (oom_score_adj < OOM_SCORE_ADJ_MIN ||
				oom_score_adj > OOM_SCORE_ADJ_MAX)
			return -EINVAL;

		task = get_proc_task(file->f_path.dentry->d_inode);
		if (!task)
			return -ESRCH;
		if (!lock_task_sighand(task, &flags)) {
			put_task_struct(task);
			return -ESRCH;
		}
		if (oom_score_adj < task->signal->oom_score_adj &&
				!capable(CAP_SYS_RESOURCE)) {
			unlock_task_sighand(task, &flags);
			put_task_struct(task);
			return -EACCES;
		}

		task->signal->oom_score_adj = oom_score_adj;

		unlock_task_sighand(task, &flags);
		put_task_struct(task);
		return count;
	}

This is just the current oom_score_adj_read() + "if (depraceted_mode)"
which does oom_adj -> oom_score_adj conversion.

Now,

	static ssize_t oom_adjust_write(...)
	{
		printk_once(KERN_WARNING "... deprecated ...\n");

		return oom_any_adj_write(..., true);
	}

	static ssize_t oom_score_adj_write(...)
	{
		return oom_any_adj_write(..., false);
	}

The same for oom_xxx_read().

What is the point to keep signal->oom_adj ?

Oleg.

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