Re: [PATCH v2 1/2][BUGFIX] mm: do not use page_count without a page pin

[Minchan Kim <minchan@kernel.org>]

Missing Bartlomiej, Sorry!

On 06/14/2012 10:12 AM, Minchan Kim wrote:

> d179e84ba fixed the problem[1] in vmscan.c but same problem is here.
> Let's fix it.
> 
> [1] http://comments.gmane.org/gmane.linux.kernel.mm/65844
> 
> I copy and paste d179e84ba's contents for description.
> 
> "It is unsafe to run page_count during the physical pfn scan because
> compound_head could trip on a dangling pointer when reading
> page->first_page if the compound page is being freed by another CPU."
> 
> * changelog from v1
>   - Add comment about skip tail page of THP - Andrea
>   - fix typo - Wanpeng Li
>   - based on next-20120613
> 
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Wanpeng Li <liwp.linux@gmail.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  mm/page_alloc.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 266f267..543cc2d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5496,11 +5496,18 @@ __count_immobile_pages(struct zone *zone, struct page *page, int count)
>  			continue;
>  
>  		page = pfn_to_page(check);
> -		if (!page_count(page)) {
> +		/*
> +		 * We can't use page_count without pin a page
> +		 * because another CPU can free compound page.
> +		 * This check already skips compound tails of THP
> +		 * because their page->_count is zero at all time.
> +		 */
> +		if (!atomic_read(&page->_count)) {
>  			if (PageBuddy(page))
>  				iter += (1 << page_order(page)) - 1;
>  			continue;
>  		}
> +
>  		if (!PageLRU(page))
>  			found++;
>  		/*



-- 
Kind regards,
Minchan Kim

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