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Date:   Wed, 14 Apr 2021 14:55:39 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Feng Tang <feng.tang@intel.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Dave Hansen <dave.hansen@intel.com>,
        Ben Widawsky <ben.widawsky@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [PATCH v4 04/13] mm/mempolicy: allow preferred code to take a
 nodemask
Message-ID: <YHbmS6R+CpSDWtAD@dhcp22.suse.cz>
References: <1615952410-36895-1-git-send-email-feng.tang@intel.com>
 <1615952410-36895-5-git-send-email-feng.tang@intel.com>
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On Wed 17-03-21 11:40:01, Feng Tang wrote:
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> Create a helper function (mpol_new_preferred_many()) which is usable
> both by the old, single-node MPOL_PREFERRED and the new
> MPOL_PREFERRED_MANY.
> 
> Enforce the old single-node MPOL_PREFERRED behavior in the "new"
> version of mpol_new_preferred() which calls mpol_new_preferred_many().
> 
> v3:
>   * fix a stack overflow caused by emty nodemask (Feng)
> 
> Link: https://lore.kernel.org/r/20200630212517.308045-5-ben.widawsky@intel.com
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
>  mm/mempolicy.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 1228d8e..6fb2cab 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -203,17 +203,34 @@ static int mpol_new_interleave(struct mempolicy *pol, const nodemask_t *nodes)
>  	return 0;
>  }
>  
> -static int mpol_new_preferred(struct mempolicy *pol, const nodemask_t *nodes)
> +static int mpol_new_preferred_many(struct mempolicy *pol,
> +				   const nodemask_t *nodes)
>  {
>  	if (!nodes)
>  		pol->flags |= MPOL_F_LOCAL;	/* local allocation */

Now you have confused me. I thought that MPOL_PREFERRED_MANY for NULL
nodemask will be disallowed as it is effectively MPOL_PREFERRED aka
MPOL_F_LOCAL. Or do I misread the code?

>  	else if (nodes_empty(*nodes))
>  		return -EINVAL;			/*  no allowed nodes */
>  	else
> -		pol->v.preferred_nodes = nodemask_of_node(first_node(*nodes));
> +		pol->v.preferred_nodes = *nodes;
>  	return 0;
>  }
>  
> +static int mpol_new_preferred(struct mempolicy *pol, const nodemask_t *nodes)
> +{
> +	if (nodes) {
> +		/* MPOL_PREFERRED can only take a single node: */
> +		nodemask_t tmp;
> +
> +		if (nodes_empty(*nodes))
> +			return -EINVAL;
> +
> +		tmp = nodemask_of_node(first_node(*nodes));
> +		return mpol_new_preferred_many(pol, &tmp);
> +	}
> +
> +	return mpol_new_preferred_many(pol, NULL);
> +}
> +
>  static int mpol_new_bind(struct mempolicy *pol, const nodemask_t *nodes)
>  {
>  	if (nodes_empty(*nodes))
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs