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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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