linux-kernel - Re: [PATCH] mm: page_alloc: place zone id check before VM_BUG_ON

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Date:	Wed, 10 Dec 2014 14:03:44 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Weijie Yang <weijie.yang.kh@...il.com>
Cc:	Weijie Yang <weijie.yang@...sung.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: place zone id check before
 VM_BUG_ON_PAGE check

On Wed, Dec 10, 2014 at 09:38:42PM +0800, Weijie Yang wrote:
> On Tue, Dec 9, 2014 at 5:59 PM, Mel Gorman <mgorman@...e.de> wrote:
> > On Tue, Dec 09, 2014 at 03:40:35PM +0800, Weijie Yang wrote:
> >> If the free page and its buddy has different zone id, the current
> >> zone->lock cann't prevent buddy page getting allocated, this could
> >> trigger VM_BUG_ON_PAGE in a very tiny chance:
> >>
> >
> > Under what circumstances can a buddy page be allocated without the
> > zone->lock? Any parallel allocation from that zone that takes place will
> > be from the per-cpu allocator and should not be affected by this. Have
> > you actually hit this race?
> 
> My description maybe not clear, if the free page and its buddy is not
> at the same zone, the holding zone->lock cann't prevent buddy page
> getting allocated.

You're right, the description is not clear but now I see your
point. Thanks.

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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