Re: [PATCH] mm: do not use page_count without a page pin

[Kamezawa Hiroyuki]

(2012/06/11 9:17), 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."
> 
> Cc: Andrea Arcangeli<aarcange@xxxxxxxxxx>
> Cc: Mel Gorman<mgorman@xxxxxxx>
> Cc: Michal Hocko<mhocko@xxxxxxx>
> Cc: KAMEZAWA Hiroyuki<kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Signed-off-by: Minchan Kim<minchan@xxxxxxxxxx>
> ---
>   mm/page_alloc.c |    6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 266f267..019c4fe 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5496,7 +5496,11 @@ __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 withou pin a page
> +		 * because another CPU can free compound page.
> +		 */
> +		if (!atomic_read(&page->_count)) {
>   			if (PageBuddy(page))
>   				iter += (1<<  page_order(page)) - 1;
>   			continue;
Nice Catch.

Other than the comment fix already pointed out..
Hmm...BTW, it seems this __count_xxx doesn't have any code for THP/Hugepage..
so, we need more fixes for better code, I think.
Hmm, Don't we need !PageTail() check and 'skip thp' code ?

Thanks,
-Kame

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