linux-kernel - Re: [PATCH] [BUGFIX] update mm->owner even if no next owner.

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Date:	Sat, 11 Jun 2011 09:46:40 +0900
From:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ying Han <yinghan@...gle.com>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] [BUGFIX] update mm->owner even if no next owner.

2011/6/11 Hugh Dickins <hughd@...gle.com>:
> On Fri, 10 Jun 2011, KAMEZAWA Hiroyuki wrote:
>>
>> I think this can be a fix.
>
> Sorry, I think not: I've not digested your rationale,
> but three things stand out:
>
> 1. Why has this only just started happening?  I may not have run that
>   test on 3.0-rc1, but surely I ran it for hours with 2.6.39;
>   maybe not with khugepaged, but certainly with ksmd.
>
Not sure. I pointed this just by review because I found "charge" in
khugepaged is out of mmap_sem now.

> 2. Your hunk below:
>> -     if (!mm_need_new_owner(mm, p))
>> +     if (!mm_need_new_owner(mm, p)) {
>> +             rcu_assign_pointer(mm->owner, NULL);
>   is now setting mm->owner to NULL at times when we were sure it did not
>   need updating before (task is not the owner): you're damaging mm->owner.
>
Ah, yes. It's my mistake.

> 3. There's a patch from Andrea in 3.0-rc1 which looks very likely to be
>   relevant, 692e0b35427a "mm: thp: optimize memcg charge in khugepaged".
>   I'll try reproducing without that tonight (I crashed in 20 minutes
>   this morning, so it's not too hard).
>

Thanks,
-Kame
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