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

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Date:   Wed, 3 Aug 2022 14:56:44 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        "akpm@...ux-foundation.org" <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>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH] mm: mempolicy: fix policy_nodemask() for
 MPOL_PREFERRED_MANY case

On Thu 04-08-22 04:43:06, Feng Tang wrote:
> On Wed, Aug 03, 2022 at 07:28:59PM +0800, Michal Hocko wrote:
[...]
> > +struct mempolicy *policy_mbind_nodemask(gfp_t gfp)
> > +{
> > +#ifdef CONFIG_MEMPOLICY
> > +	struct mempolicy *mpol = get_task_policy(current);
> > +
> > +	/*
> > +	 * only enforce MBIND which overlaps with cpuset policy (from policy_nodemask)
> > +	 * specifically for hugetlb case
> > +	 */
> > +	if (mpol->mode == MPOL_BIND &&
> > +		(apply_policy_zone(mpol, gfp_zone(gfp)) &&
> > +		 cpuset_nodemask_valid_mems_allowed(&policy->nodes))
> > +		return &mpol->nodes;
> > +#endif
> > +	return NULL;
> 
> I saw the logic is not changed, and it confused me that if there is
> no qualified node, it will still return NULL which effectively equals
> node_states[N_MEMORY], while I think it should return a all zero
> nodemasks.

This is a separate thing and I have to admit that the existing code is
rather non-intuitive or even broken. I guess we do not care all that
much because MBIND with completely non-overlapping cpusets is just a
broken configuration. I am not sure this case is interesting or even
supported.

-- 
Michal Hocko
SUSE Labs