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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Subject: Re: [patch -mm 15/18] oom: introduce find_lock_task_mm() to fix !mm false positives
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References: <alpine.DEB.2.00.1006010008410.29202@chino.kir.corp.google.com> <alpine.DEB.2.00.1006010017090.29202@chino.kir.corp.google.com>
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Date: Tue,  1 Jun 2010 16:41:30 +0900 (JST)
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To: David Rientjes <rientjes@google.com>
Cc: kosaki.motohiro@jp.fujitsu.com, Andrew Morton <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>, Nick Piggin <npiggin@suse.de>, Oleg Nesterov <oleg@redhat.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Balbir Singh <balbir@linux.vnet.ibm.com>, linux-mm@kvack.org
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> From: Oleg Nesterov <oleg@redhat.com>
> 
> 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 oom_badness() returns the
>   task can be chosen if nothing else has been found yet.
> 
>   Change oom_badness() to return int, change it to return -1 if
>   find_lock_task_mm() fails, and change select_bad_process() to
>   check points >= 0.
> 
> Note! This patch is not enough, we need more changes.
> 
> 	- oom_badness() was fixed, but oom_kill_task() still ignores
> 	  the task without ->mm
> 
> 	- oom_forkbomb_penalty() should use find_lock_task_mm() too,
> 	  and it also needs other changes to actually find the first
> 	  first-descendant children
> 
> This will be addressed later.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: David Rientjes <rientjes@google.com>

need respin.



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