On Tue, Apr 26, 2016 at 02:04:51PM +0200, Vlastimil Babka wrote: > 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? > You're right, this is a potential problem. I thought of a few solutions but they're not necessarily cheaper than the current code. If Andrew is watching, please drop this patch if possible. Otherwise, I'll post a revert within the next 2 days and find an alternative solution that still saves cycles. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>