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

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Message-ID: <YueXhmiFcI8iw3OI@dhcp22.suse.cz>
Date:   Mon, 1 Aug 2022 11:06:14 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     akpm@...ux-foundation.org, bwidawsk@...nel.org,
        dave.hansen@...ux.intel.com, feng.tang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: mempolicy: fix policy_nodemask() for
 MPOL_PREFERRED_MANY case

On Mon 01-08-22 16:42:07, Muchun Song wrote:
> policy_nodemask() is supposed to be returned a nodemask representing a mempolicy
> for filtering nodes for page allocation, which is a hard restriction (see the user
> of allowed_mems_nr() in hugetlb.c).  However, MPOL_PREFERRED_MANY is a preferred
> mode not a hard restriction.  Now it breaks the user of HugeTLB.  Remove it from
> policy_nodemask() to fix it, which will not affect current users of policy_nodemask()
> since all of the users already have handled the case of MPOL_PREFERRED_MANY before
> calling it.  BTW, it is found by code inspection.

I am not sure this is the right fix. It is quite true that
policy_nodemask is a tricky function to use. It pretends to have a
higher level logic but all existing users are expected to be policy
aware and they special case allocation for each policy. That would mean
that hugetlb should do the same.

I haven't checked the actual behavior implications for hugetlb here. Is
MPOL_PREFERRED_MANY even supported for hugetlb? Does this change make it
work? From a quick look this just ignores MPOL_PREFERRED_MANY
completely.
 
> Fixes: b27abaccf8e8 ("mm/mempolicy: add MPOL_PREFERRED_MANY for multiple preferred nodes")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/mempolicy.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 6c27acb6cd63..4deec7e598c6 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1845,9 +1845,6 @@ nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
>  		cpuset_nodemask_valid_mems_allowed(&policy->nodes))
>  		return &policy->nodes;
>  
> -	if (mode == MPOL_PREFERRED_MANY)
> -		return &policy->nodes;
> -
>  	return NULL;
>  }
>  
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs