linux-kernel - Re: Need help in bug in isolate_migratepages

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Date:	Mon, 3 Feb 2014 16:06:51 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
cc:	Michal Hocko <mhocko@...e.cz>, Holger Kiehl <Holger.Kiehl@....de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org
Subject: Re: Need help in bug in isolate_migratepages_range

On Mon, 3 Feb 2014, Vlastimil Babka wrote:

> It seems to come from balloon_page_movable() and its test page_count(page) ==
> 1.
> 

Hmm, I think it might be because compound_head() == NULL here.  Holger, 
this looks like a race condition when allocating a compound page, did you 
only see it once or is it actually reproducible?

I think this happens when a new compound page is allocated and PageBuddy 
is cleared before prep_compound_page() and then we see PageTail(p) set but 
p->first_page is not yet initialized.  Is there any way to avoid memory 
barriers in compound_page()?
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