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

[Wanpeng Li]

On Mon, Jun 11, 2012 at 09:17:51AM +0900, 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
                                        ^
										without
>+		 * because another CPU can free compound page.
>+		 */
>+		if (!atomic_read(&page->_count)) {
> 			if (PageBuddy(page))
> 				iter += (1 << page_order(page)) - 1;
> 			continue;
>-- 
>1.7.9.5
>
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