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

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