linux-kernel - Re: [PATCH] mm: mempolicy: fix policy_nodemask() for MPOL_PREFERRED

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Message-ID: <YuuitMLOYqBcXMR3@dhcp22.suse.cz>
Date:   Thu, 4 Aug 2022 12:43:00 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "bwidawsk@...nel.org" <bwidawsk@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: mempolicy: fix policy_nodemask() for
 MPOL_PREFERRED_MANY case

On Thu 04-08-22 16:27:24, Feng Tang wrote:
[...]
> >From a2db9a57da616bb3ea21e48a4a9ceb5c2cf4f7a2 Mon Sep 17 00:00:00 2001
> From: Feng Tang <feng.tang@...el.com>
> Date: Thu, 4 Aug 2022 09:39:24 +0800
> Subject: [PATCH RFC] mm/hugetlb: add dedicated func to get 'allowed' nodemask for
>  current process
> 
> 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().
> 
> Michal found the policy_nodemask_current() is only used by hugetlb,
> and suggested to move it to hugetlb code with more explicit name to
> enforce the 'allowed' semantics for which only MPOL_BIND policy
> matters.
> 
> One note for the new policy_mbind_nodemask() is, the cross check
> from MPOL_BIND, gfp flags and cpuset configuration can lead to
> a no available node case, which is considered to be broken
> configuration and 'NULL' (equals all nodes) is returned.
> 
> [1]. https://lore.kernel.org/lkml/20220801084207.39086-1-songmuchun@bytedance.com/t/
> Reported-by: Muchun Song <songmuchun@...edance.com>
> Suggested-by: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Feng Tang <feng.tang@...el.com>

LGTM I would just make apply_policy_zone extern rather than making it
static inline in a header which can turn out to cause other header
dependencies.

Thanks!
-- 
Michal Hocko
SUSE Labs