linux-kernel - Re: [patch] mm, page_alloc: make first

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Message-ID: <alpine.DEB.2.02.1402041613450.14962@chino.kir.corp.google.com>
Date:	Tue, 4 Feb 2014 16:14:07 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Holger Kiehl <Holger.Kiehl@....de>,
	Christoph Lameter <cl@...ux.com>,
	Rafael Aquini <aquini@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Mel Gorman <mgorman@...e.de>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, page_alloc: make first_page visible before
 PageTail

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@....de>
> 
> What did Holger report?
> 

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