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
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