linux-kernel - Re: setup_per_zone_pages_min(): zone->lock vs. zone->lru

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Message-Id: <1222723206.6791.2.camel@ubuntu>
Date:	Mon, 29 Sep 2008 23:20:05 +0200
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	Andy Whitcroft <apw@...dowen.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: setup_per_zone_pages_min(): zone->lock vs. zone->lru_lock

On Mon, 2008-09-29 at 18:36 +0100, Andy Whitcroft wrote:
> The allocator protects it freelists using zone->lock (as we can see in
> rmqueue_bulk), so anything which manipulates those should also be using
> that lock.  move_freepages() is scanning the cmap and picking up free
> pages directly off the free lists, it is expecting those lists to be
> stable; it would appear to need zone->lock.  It does look like
> setup_per_zone_pages_min() is holding the wrong thing at first look.

I just noticed that the spin_lock in that function is much older than the
call to setup_zone_migrate_reserve(), which then calls move_freepages().
So it seems that the zone->lru_lock there does (did?) have another purpose,
maybe protecting zone->present_pages/pages_min/etc.

Looks like the need for a zone->lock (if any) was added later, but I'm not
sure if makes sense to take both locks together, or if the lru_lock is still
needed at all.

Thanks,
Gerald


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