Re: [PATCH] proc: put check_mem_permission before __get_free_page in mem_read

[Stephen Wilson <wilsons@start.ca>]

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@jp.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@gmail.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Stephen Wilson <wilsons@start.ca>


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

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

-- 
steve