Re: [PATCH v2] mm/hugetlb: add dedicated func to get 'allowed' nodemask for current process

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Aug 05, 2022 at 08:59:03AM +0800, Feng Tang wrote:
> Muchun Song found that after MPOL_PREFERRED_MANY policy was introduced
> in commit b27abaccf8e8 ("mm/mempolicy: add MPOL_PREFERRED_MANY for multiple preferred nodes"),
> the policy_nodemask_current()'s semantics for this new policy has been
> changed, which returns 'preferred' nodes instead of 'allowed' nodes.
> 
> With the changed semantic of policy_nodemask_current, a task with
> MPOL_PREFERRED_MANY policy could fail to get its reservation even though
> it can fall back to other nodes (either defined by cpusets or all online
> nodes) for that reservation failing mmap calles unnecessarily early.
> 
> The fix is to not consider MPOL_PREFERRED_MANY for reservations at all
> because they, unlike MPOL_MBIND, do not pose any actual hard constrain.
> 
> Michal suggested the policy_nodemask_current() is only used by hugetlb,
> and could be moved to hugetlb code with more explicit name to enforce
> the 'allowed' semantics for which only MPOL_BIND policy matters.
> 
> apply_policy_zone() is made extern to be called in hugetlb code
> and its return value is changed to bool.
> 
> [1]. https://lore.kernel.org/lkml/20220801084207.39086-1-songmuchun@bytedance.com/t/
> 
> Fixes: b27abaccf8e8 ("mm/mempolicy: add MPOL_PREFERRED_MANY for multiple preferred nodes")
> Reported-by: Muchun Song <songmuchun@bytedance.com>
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks for fixing this.

Reviewed-by: Muchun Song <songmuchun@bytedance.com>