linux-kernel - Re: [PATCH] proc: put check_mem_permission before __get_free_page in mem

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Date:	Tue, 26 Apr 2011 12:03:00 -0400
From:	Stephen Wilson <wilsons@...rt.ca>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Hugh Dickins <hughd@...gle.com>, bookjovi@...il.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Rientjes <rientjes@...gle.com>,
	Stephen Wilson <wilsons@...rt.ca>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] proc: put check_mem_permission before __get_free_page
	in mem_read

On Tue, Apr 26, 2011 at 02:12:46PM +0900, KOSAKI Motohiro wrote:
> From 74f827ce74e1c4f846905e940edfa5f639b5a2ce Mon Sep 17 00:00:00 2001
> From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Date: Tue, 26 Apr 2011 13:57:02 +0900
> Subject: [PATCH] [PATCH] proc: put check_mem_permission after __get_free_page in mem_write
> 
> It should be better if put check_mem_permission after __get_free_page
> in mem_write, to be same as function mem_read.
> 
> Hugh Dickins explained the reason.
> 
>     check_mem_permission gets a reference to the mm.  If we __get_free_page
>     after check_mem_permission, imagine what happens if the system is out
>     of memory, and the mm we're looking at is selected for killing by the
>     OOM killer: while we wait in __get_free_page for more memory, no memory
>     is freed from the selected mm because it cannot reach exit_mmap while
>     we hold that reference.
> 
> 
> Reported-by: Jovi Zhang <bookjovi@...il.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Stephen Wilson <wilsons@...rt.ca>


I completely missed the oom case as well.  This certainly appears to be
the best solution.

	Reviewed-by: Stephen Wilson <wilsons@...rt.ca>

-- 
steve

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