linux-kernel - Re: [PATCH -mm] select_bad_process: cleanup 'releasing' check

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Date:	Mon, 28 Aug 2006 12:44:59 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm] select_bad_process: cleanup 'releasing' check

On Sun, Aug 27, 2006 at 10:25:38PM +0400, Oleg Nesterov wrote:
> On top of "select_bad_process: kill a bogus PF_DEAD/TASK_DEAD check"
> 
> No logic changes, but imho easier to read.
> 
> Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
> 
> --- 2.6.18-rc4/mm/oom_kill.c~	2006-08-27 20:56:23.000000000 +0400
> +++ 2.6.18-rc4/mm/oom_kill.c	2006-08-27 21:58:32.000000000 +0400
> @@ -205,7 +205,6 @@ static struct task_struct *select_bad_pr
>  	do_posix_clock_monotonic_gettime(&uptime);
>  	do_each_thread(g, p) {
>  		unsigned long points;
> -		int releasing;
>  
>  		/*
>  		 * skip kernel threads and tasks which have already released
> @@ -227,16 +226,15 @@ static struct task_struct *select_bad_pr
>  		 * the process of exiting and releasing its resources.
>  		 * Otherwise we could get an OOM deadlock.
>  		 */
> -		releasing = test_tsk_thread_flag(p, TIF_MEMDIE) ||
> -						p->flags & PF_EXITING;
> -		if (releasing) {
> -			if (p->flags & PF_EXITING && p == current) {
> -				chosen = p;
> -				*ppoints = ULONG_MAX;
> -				break;
> -			}
> -			return ERR_PTR(-1UL);
> -		}
> +		if ((p->flags & PF_EXITING) && p == current) {
> +			chosen = p;
> +			*ppoints = ULONG_MAX;
> +			break;
> +		}
> +		if ((p->flags & PF_EXITING) ||
> +				test_tsk_thread_flag(p, TIF_MEMDIE))
> +			return ERR_PTR(-1UL);
> +

Hmm, actually I think I spot a bug in the original logic: we don't want
to have more than 1 task with TIF_MEMDIE at once, becaues that gives it
access to memory reserves (but I saw it first in the new formulation, so
maybe that does suggest it is more readable ;)

What I think should be done is the check for TIF_MEMDIE (and return -1)
first, and then the PF_EXITING test. At which point, if current is found to
be exiting, it should be chosen but not break... that way a subsequent MEMDIE
or EXITING task has the chance to trigger the -1 return.

Anyway, if you don't want to do all that, I will when my hand gets better.
Otherwise the 3 patches you sent look good, they could all have an

Acked-by: Nick Piggin <npiggin@...e.de>

Thanks,
Nick
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