Re: [PATCH 3/5] oom: introduce find_lock_task_mm() to fix !mm false positives

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 31 May 2010 18:36:34 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> From: Oleg Nesterov <oleg@redhat.com>
> Subject: [PATCH 3/5] oom: introduce find_lock_task_mm() to fix !mm false positives
> 
> Almost all ->mm == NUL checks in oom_kill.c are wrong.
> 
> The current code assumes that the task without ->mm has already
> released its memory and ignores the process. However this is not
> necessarily true when this process is multithreaded, other live
> sub-threads can use this ->mm.
> 
> - Remove the "if (!p->mm)" check in select_bad_process(), it is
>   just wrong.
> 
> - Add the new helper, find_lock_task_mm(), which finds the live
>   thread which uses the memory and takes task_lock() to pin ->mm
> 
> - change oom_badness() to use this helper instead of just checking
>   ->mm != NULL.
> 
> - As David pointed out, select_bad_process() must never choose the
>   task without ->mm, but no matter what badness() returns the
>   task can be chosen if nothing else has been found yet.
> 
> Note! This patch is not enough, we need more changes.
> 
> 	- badness() was fixed, but oom_kill_task() still ignores
> 	  the task without ->mm
> 
> This will be addressed later.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Cc: David Rientjes <rientjes@google.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [rebase
> latest -mm and remove some obsoleted description]

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>