Re: [RFC 1/4] mm, page_alloc: fix check for NULL preferred_zone

[Vlastimil Babka <vbabka@suse.cz>]

On 01/18/2017 10:31 AM, Michal Hocko wrote:
> On Tue 17-01-17 23:16:07, Vlastimil Babka wrote:
>> Since commit c33d6c06f60f ("mm, page_alloc: avoid looking up the first zone in
>> a zonelist twice") we have a wrong check for NULL preferred_zone, which can
>> theoretically happen due to concurrent cpuset modification. We check the
>> zoneref pointer which is never NULL and we should check the zone pointer.
>>
>> Fixes: c33d6c06f60f ("mm, page_alloc: avoid looking up the first zone in a zonelist twice")
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>>  mm/page_alloc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 34ada718ef47..593a11d8bc6b 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3763,7 +3763,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>>  	 */
>>  	ac.preferred_zoneref = first_zones_zonelist(ac.zonelist,
>>  					ac.high_zoneidx, ac.nodemask);
>> -	if (!ac.preferred_zoneref) {
>> +	if (!ac.preferred_zoneref->zone) {
>
> When can the ->zone be NULL?

Either we get a genuinely screwed nodemask, or there's a concurrent cpuset 
update and nodes in zonelist are ordered in such a way that we see all of them 
as not being available to us in the nodemask/current->mems_alowed, when we 
iterate the zonelist, so we reach the end of zonelist. The zonelists are 
terminated with a zoneref with NULL zone pointer.

>
>>  		page = NULL;
>>  		goto no_zone;
>>  	}
>> --
>> 2.11.0
>

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