Re: [PATCH 13/28] mm, page_alloc: Remove redundant check for empty zonelist

[Vlastimil Babka]

On 04/15/2016 11:07 AM, Mel Gorman wrote:
> A check is made for an empty zonelist early in the page allocator fast path
> but it's unnecessary. When get_page_from_freelist() is called, it'll return
> NULL immediately. Removing the first check is slower for machines with
> memoryless nodes but that is a corner case that can live with the overhead.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>   mm/page_alloc.c | 11 -----------
>   1 file changed, 11 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index df03ccc7f07c..21aaef6ddd7a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3374,14 +3374,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>   	if (should_fail_alloc_page(gfp_mask, order))
>   		return NULL;
>
> -	/*
> -	 * Check the zones suitable for the gfp_mask contain at least one
> -	 * valid zone. It's possible to have an empty zonelist as a result
> -	 * of __GFP_THISNODE and a memoryless node
> -	 */
> -	if (unlikely(!zonelist->_zonerefs->zone))
> -		return NULL;
> -
>   	if (IS_ENABLED(CONFIG_CMA) && ac.migratetype == MIGRATE_MOVABLE)
>   		alloc_flags |= ALLOC_CMA;
>
> @@ -3394,8 +3386,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>   	/* The preferred zone is used for statistics later */
>   	preferred_zoneref = first_zones_zonelist(ac.zonelist, ac.high_zoneidx,
>   				ac.nodemask, &ac.preferred_zone);
> -	if (!ac.preferred_zone)
> -		goto out;

Is this part really safe? Besides changelog doesn't mention preferred_zone. What 
if somebody attempts e.g. a DMA allocation with ac.nodemask being set to 
cpuset_current_mems_allowed and initially only containing nodes without 
ZONE_DMA. Then ac.preferred_zone is NULL, yet we proceed to 
get_page_from_freelist(). Meanwhile cpuset_current_mems_allowed gets changed so 
in fact it does contains a suitable node, so we manage to get inside 
for_each_zone_zonelist_nodemask(). Then there's zone_local(ac->preferred_zone, 
zone), which will defererence the NULL ac->preferred_zone?

>   	ac.classzone_idx = zonelist_zone_idx(preferred_zoneref);
>
>   	/* First allocation attempt */
> @@ -3418,7 +3408,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>
>   	trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);
>
> -out:
>   	/*
>   	 * When updating a task's mems_allowed, it is possible to race with
>   	 * parallel threads in such a way that an allocation can fail while