linux-kernel - Re: [patch v2] vmscan: protect zone rotation stats by lru lock

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Message-ID: <49345B3B.30703@redhat.com>
Date:	Mon, 01 Dec 2008 16:46:35 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Johannes Weiner <hannes@...urebad.de>,
	torvalds@...ux-foundation.org, kosaki.motohiro@...fujitsu.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch v2] vmscan: protect zone rotation stats by lru lock

Andrew Morton wrote:
> On Mon, 01 Dec 2008 03:00:35 +0100
> Johannes Weiner <hannes@...urebad.de> wrote:
> 
>> The zone's rotation statistics must not be accessed without the
>> corresponding LRU lock held.  Fix an unprotected write in
>> shrink_active_list().
>>
> 
> I don't think it really matters.  It's quite common in that code to do
> unlocked, racy update to statistics such as this.  Because on those
> rare occasions where a race does happen, there's a small glitch in the
> reclaim logic which nobody will notice anyway.
> 
> Of course, this does need to be done with some care, to ensure the
> glitch _will_ be small.

Processing at most SWAP_CLUSTER_MAX pages at once probably
ensures that glitches will be small most of the time.

The only way this could be a big problem is if we end up
racing with the divide-by-two logic in get_scan_ratio,
leaving the rotated pages a factor two higher than they
should be.

Putting all the writes to the stats under the LRU lock
should ensure that never happens.

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