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 <mgor...@techsingularity.net>
---
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
