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    SubjectRe: [PATCH] page_alloc: skip cpuset enforcement for lower zone allocations
    On Fri, 23 May 2014, Marcelo Tosatti wrote:

    > Zone specific allocations, such as GFP_DMA32, should not be restricted
    > to cpusets allowed node list: the zones which such allocations demand
    > might be contained in particular nodes outside the cpuset node list.
    >
    > The alternative would be to not perform such allocations from
    > applications which are cpuset restricted, which is unrealistic.
    >

    Or ensure applications that allocate from lowmem are allowed to do so, but
    I understand that might be hard to make sure always happens.

    > Fixes KVM's alloc_page(gfp_mask=GFP_DMA32) with cpuset as explained.
    >
    > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
    >
    > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > index 5dba293..f228039 100644
    > --- a/mm/page_alloc.c
    > +++ b/mm/page_alloc.c
    > @@ -2698,6 +2698,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
    > unsigned int cpuset_mems_cookie;
    > int alloc_flags = ALLOC_WMARK_LOW|ALLOC_CPUSET|ALLOC_FAIR;
    > struct mem_cgroup *memcg = NULL;
    > + nodemask_t *cpuset_mems_allowed = &cpuset_current_mems_allowed;
    >
    > gfp_mask &= gfp_allowed_mask;
    >
    > @@ -2726,9 +2727,14 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
    > retry_cpuset:
    > cpuset_mems_cookie = read_mems_allowed_begin();
    >
    > +#ifdef CONFIG_NUMA
    > + if (gfp_zone(gfp_mask) < policy_zone)
    > + cpuset_mems_allowed = NULL;
    > +#endif
    > +
    > /* The preferred zone is used for statistics later */
    > first_zones_zonelist(zonelist, high_zoneidx,
    > - nodemask ? : &cpuset_current_mems_allowed,
    > + nodemask ? : cpuset_mems_allowed,
    > &preferred_zone);
    > if (!preferred_zone)
    > goto out;
    >

    I think this is incomplete. Correct me if I'm wrong on how this is
    working: preferred_zone, today, is NULL because first_zones_zonelist() is
    restricted to a cpuset.mems that does not include lowmem and your patch
    fixes that. But if the fastpath allocation with mandatory ALLOC_CPUSET
    fails and we go to the slowpath, which may or may not have showed up in
    your testing, there's still issues, particularly if __GFP_WAIT and lots of
    allocators do GFP_KERNEL | __GFP_DMA32. This requires ALLOC_CPUSET on all
    allocations and you haven't updated __cpuset_node_allowed_softwall() with
    this exception nor zlc_setup().

    After that's done, I think all of this is really convoluted and deserves a
    comment to describe the ALLOC_CPUSET and __GFP_DMA32 behavior.

    Adding Li, the cpusets maintainer, to this as well.


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