Date: Tue, 4 Feb 2014 16:14:07 -0800 (PST)
From: David Rientjes <rientjes@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
cc: Holger Kiehl <Holger.Kiehl@dwd.de>, Christoph Lameter <cl@linux.com>,
        Rafael Aquini <aquini@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.cz>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch] mm, page_alloc: make first_page visible before
 PageTail
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References: <alpine.LRH.2.02.1401312037340.6630@diagnostix.dwd.de> <20140203122052.GC2495@dhcp22.suse.cz> <alpine.LRH.2.02.1402031426510.13382@diagnostix.dwd.de> <20140203162036.GJ2495@dhcp22.suse.cz> <52EFC93D.3030106@suse.cz>
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On Tue, 4 Feb 2014, Andrew Morton wrote:

> > Commit bf6bddf1924e ("mm: introduce compaction and migration for ballooned 
> > pages") introduces page_count(page) into memory compaction which 
> > dereferences page->first_page if PageTail(page).
> > 
> > Introduce a store memory barrier to ensure page->first_page is properly 
> > initialized so that code that does page_count(page) on pages off the lru 
> > always have a valid p->first_page.
> 
> Could we have a code comment please?  Even checkpatch knows this rule!
> 

Ok.

> > Reported-by: Holger Kiehl <Holger.Kiehl@dwd.de>
> 
> What did Holger report?
> 

A once-in-five-years NULL pointer dereference on the aforementioned 
page_count(page).
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