linux-kernel - Re: [patch v2] mm, page_alloc: make first

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Date:	Wed, 5 Feb 2014 09:42:35 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Holger Kiehl <Holger.Kiehl@....de>,
	Christoph Lameter <cl@...ux.com>,
	Rafael Aquini <aquini@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v2] mm, page_alloc: make first_page visible before
 PageTail

On Tue 04-02-14 16:22:53, David Rientjes 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.
> 
> Reported-by: Holger Kiehl <Holger.Kiehl@....de>
> Signed-off-by: David Rientjes <rientjes@...gle.com>
> ---
>  v2: with commentary, per checkpatch
> 
>  mm/page_alloc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -369,9 +369,11 @@ void prep_compound_page(struct page *page, unsigned long order)
>  	__SetPageHead(page);
>  	for (i = 1; i < nr_pages; i++) {
>  		struct page *p = page + i;
> -		__SetPageTail(p);
>  		set_page_count(p, 0);
>  		p->first_page = page;
> +		/* Make sure p->first_page is always valid for PageTail() */
> +		smp_wmb();
> +		__SetPageTail(p);

Where is the pairing smp_rmb? I would expect it in comound_head.

>  	}
>  }
>  

-- 
Michal Hocko
SUSE Labs
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