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

Kamezawa Hiroyuki
2012-06-11T07:22:30Z

(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@redhat.com>
> Cc: Mel Gorman<mgorman@suse.de>
> Cc: Michal Hocko<mhocko@suse.cz>
> Cc: KAMEZAWA Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: Minchan Kim<minchan@kernel.org>
> ---
> 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