Re: [PATCH 2/4] mm: prevent double decrease of nr_reserved_highatomic

[Vlastimil Babka <vbabka@suse.cz>]

On 10/07/2016 07:45 AM, Minchan Kim wrote:
> There is race between page freeing and unreserved highatomic.
>
>  CPU 0				    CPU 1
>
>     free_hot_cold_page
>       mt = get_pfnblock_migratetype

so here mt == MIGRATE_HIGHATOMIC?

>       set_pcppage_migratetype(page, mt)
>     				    unreserve_highatomic_pageblock
>     				    spin_lock_irqsave(&zone->lock)
>     				    move_freepages_block
>     				    set_pageblock_migratetype(page)
>     				    spin_unlock_irqrestore(&zone->lock)
>       free_pcppages_bulk
>         __free_one_page(mt) <- mt is stale
>
> By above race, a page on CPU 0 could go non-highorderatomic free list
> since the pageblock's type is changed.
> By that, unreserve logic of
> highorderatomic can decrease reserved count on a same pageblock
> several times and then it will make mismatch between
> nr_reserved_highatomic and the number of reserved pageblock.

Hmm I see.

> So, this patch verifies whether the pageblock is highatomic or not
> and decrease the count only if the pageblock is highatomic.

Yeah I guess that's the easiest solution.

> Signed-off-by: Minchan Kim <minchan@kernel.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e7cbb3cc22fa..d110cd640264 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2133,13 +2133,25 @@ static void unreserve_highatomic_pageblock(const struct alloc_context *ac)
>  				continue;
>
>  			/*
> -			 * It should never happen but changes to locking could
> -			 * inadvertently allow a per-cpu drain to add pages
> -			 * to MIGRATE_HIGHATOMIC while unreserving so be safe
> -			 * and watch for underflows.
> +			 * In page freeing path, migratetype change is racy so
> +			 * we can counter several free pages in a pageblock
> +			 * in this loop althoug we changed the pageblock type
> +			 * from highatomic to ac->migratetype. So we should
> +			 * adjust the count once.
>  			 */
> -			zone->nr_reserved_highatomic -= min(pageblock_nr_pages,
> -				zone->nr_reserved_highatomic);
> +			if (get_pageblock_migratetype(page) ==
> +							MIGRATE_HIGHATOMIC) {
> +				/*
> +				 * It should never happen but changes to
> +				 * locking could inadvertently allow a per-cpu
> +				 * drain to add pages to MIGRATE_HIGHATOMIC
> +				 * while unreserving so be safe and watch for
> +				 * underflows.
> +				 */
> +				zone->nr_reserved_highatomic -= min(
> +						pageblock_nr_pages,
> +						zone->nr_reserved_highatomic);
> +			}
>
>  			/*
>  			 * Convert to ac->migratetype and avoid the normal
>