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Subject: Re: setup_per_zone_pages_min(): zone->lock vs. zone->lru_lock
From: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Reply-To: gerald.schaefer@de.ibm.com
To: Andy Whitcroft <apw@shadowen.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, schwidefsky@de.ibm.com,
       heiko.carstens@de.ibm.com,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Yasunori Goto <y-goto@jp.fujitsu.com>, Mel Gorman <mel@csn.ul.ie>,
       Andrew Morton <akpm@linux-foundation.org>
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Date: Mon, 29 Sep 2008 23:20:05 +0200
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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