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

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Message-Id: <20220804153717.eea592a171accd245a0cc7d8@linux-foundation.org>
Date:   Thu, 4 Aug 2022 15:37:17 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Feng Tang <feng.tang@...el.com>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Dave Hansen <hansen.dave@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: add dedicated func to get 'allowed'
 nodemask for current process

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")?