linux-kernel - Re: [PATCH 9/10] mm/memcg: move lru

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Date:	Wed, 22 Feb 2012 07:43:20 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 9/10] mm/memcg: move lru_lock into lruvec

Hugh Dickins wrote:
> On Wed, 22 Feb 2012, Konstantin Khlebnikov wrote:
>> Hugh Dickins wrote:
>>>
>>> I'll have to come back to think about your locking later too;
>>> or maybe that's exactly where I need to look, when investigating
>>> the mm_inline.h:41 BUG.
>>
>> pages_count[] updates looks correct.
>> This really may be bug in locking, and this VM_BUG_ON catch it before
>> list-debug.
>
> I've still not got into looking at it yet.
>
> You're right to mention DEBUG_LIST: I have that on some of the machines,
> and I would expect that to be the first to catch a mislocking issue.
>
> In the past my problems with that BUG (well, the spur to introduce it)
> came from hugepages.

My patchset hasn't your mem_cgroup_reset_uncharged_to_root protection,
or something to replace it. So, there exist race between cgroup remove and
isolated uncharged page put-back, but it shouldn't corrupt lru lists.
There something different.

>
>>>
>>> But at first sight, I have to say I'm very suspicious: I've never found
>>> PageLRU a good enough test for whether we need such a lock, because of
>>> races with those pages on percpu lruvec about to be put on the lru.
>>>
>>> But maybe once I look closer, I'll find that's handled by your changes
>>> away from lruvec; though I'd have thought the same issue exists,
>>> independent of whether the pending pages are in vector or list.
>>
>> Are you talking about my per-cpu page-lists for lru-adding?
>
> Yes.
>
>> This is just an unnecessary patch, I don't know why I include it into v2 set.
>> It does not protect anything.
>
> Okay.
>
> Hugh

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