On Fri 02-12-16 11:29:50, Mel Gorman wrote:
> Vlastimil Babka pointed out that commit 479f854a207c ("mm, page_alloc:
> defer debugging checks of pages allocated from the PCP") will allow the
> per-cpu list counter to be out of sync with the per-cpu list contents
> if a struct page is corrupted.
>
> The consequence is an infinite loop if the per-cpu lists get fully drained
> by free_pcppages_bulk because all the lists are empty but the count is
> positive. The infinite loop occurs here
>
> do {
> batch_free++;
> if (++migratetype == MIGRATE_PCPTYPES)
> migratetype = 0;
> list = &pcp->lists[migratetype];
> } while (list_empty(list));
>
> >From a user perspective, it's a bad page warning followed by a soft lockup
> with interrupts disabled in free_pcppages_bulk().
>
> This patch keeps the accounting in sync.
>
> Fixes: 479f854a207c ("mm, page_alloc: defer debugging checks of pages allocated from the PCP")
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> cc: stable@xxxxxxxxxxxxxxx [4.7+]
Thanks for adding the comment it should really make the code more clear.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/page_alloc.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6de9440e3ae2..34ada718ef47 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2192,7 +2192,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
> unsigned long count, struct list_head *list,
> int migratetype, bool cold)
> {
> - int i;
> + int i, alloced = 0;
>
> spin_lock(&zone->lock);
> for (i = 0; i < count; ++i) {
> @@ -2217,13 +2217,21 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
> else
> list_add_tail(&page->lru, list);
> list = &page->lru;
> + alloced++;
> if (is_migrate_cma(get_pcppage_migratetype(page)))
> __mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
> -(1 << order));
> }
> +
> + /*
> + * i pages were removed from the buddy list even if some leak due
> + * to check_pcp_refill failing so adjust NR_FREE_PAGES based
> + * on i. Do not confuse with 'alloced' which is the number of
> + * pages added to the pcp list.
> + */
> __mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));
> spin_unlock(&zone->lock);
> - return i;
> + return alloced;
> }
>
> #ifdef CONFIG_NUMA
> --
> 2.10.2
>
--
Michal Hocko
SUSE Labs
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