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

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Date:   Fri, 5 Aug 2022 08:06:30 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     "Hocko, Michal" <mhocko@...e.com>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/hugetlb: add dedicated func to get 'allowed' nodemask
 for current process

On Fri, Aug 05, 2022 at 06:37:17AM +0800, Andrew Morton wrote:
> On Thu, 4 Aug 2022 15:36:48 +0200 Michal Hocko <mhocko@...e.com> wrote:
> 
> > On Thu 04-08-22 21:03:42, 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")
> > > [1], the policy_nodemask_current()'s semantics for this new policy
> > > has been changed, which returns 'preferred' nodes instead of 'allowed'
> > > nodes, and could hurt the usage of its caller in hugetlb:
> > > allowed_mems_nr().
> > 
> > The acutal user visible effect description is missing here. AFAIU it
> > would be this.
> > 
> > With the changed semantic of policy_nodemask_current a taks 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.
> 
> And is this Fixes: b27abaccf8e8 ("mm/mempolicy: add MPOL_PREFERRED_MANY
> for multiple preferred nodes")?

Yes. Will add it in the next version, thanks.

- Feng