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

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  [RFC 1/4] mm, page_alloc: fix check for NULL preferred_zone Vlastimil Babka <vbabka@suse.cz> - 2017-01-18 01:10 +0100
    Re: [RFC 1/4] mm, page_alloc: fix check for NULL preferred_zone Michal Hocko <mhocko@kernel.org> - 2017-01-18 10:40 +0100
      Re: [RFC 1/4] mm, page_alloc: fix check for NULL preferred_zone Vlastimil Babka <vbabka@suse.cz> - 2017-01-18 10:50 +0100
        Re: [RFC 1/4] mm, page_alloc: fix check for NULL preferred_zone Michal Hocko <mhocko@kernel.org> - 2017-01-18 11:20 +0100
    Re: [RFC 1/4] mm, page_alloc: fix check for NULL preferred_zone Mel Gorman <mgorman@techsingularity.net> - 2017-01-18 10:50 +0100

#1561151 — [RFC 1/4] mm, page_alloc: fix check for NULL preferred_zone

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) {
 		page = NULL;
 		goto no_zone;
 	}
-- 
2.11.0

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#1561397

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?

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

-- 
Michal Hocko
SUSE Labs

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#1561416

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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#1561443

On Wed 18-01-17 10:45:33, Vlastimil Babka wrote:
> 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.

Thanks for the clarification.  Please add a big fat comment in
first_zones_zonelist about this potential case.

-- 
Michal Hocko
SUSE Labs

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#1561406

On Tue, Jan 17, 2017 at 11:16:07PM +0100, 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>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs

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