linux-kernel - Re: [PATCH v6 1/6] mm/mempolicy: Add MPOL_PREFERRED

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Date:   Wed, 28 Jul 2021 14:31:03 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>, ying.huang@...el.com,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v6 1/6] mm/mempolicy: Add MPOL_PREFERRED_MANY for
 multiple preferred nodes

[Sorry for a late review]

On Mon 12-07-21 16:09:29, Feng Tang wrote:
[...]
> @@ -1887,7 +1909,8 @@ nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
>  /* Return the node id preferred by the given mempolicy, or the given id */
>  static int policy_node(gfp_t gfp, struct mempolicy *policy, int nd)
>  {
> -	if (policy->mode == MPOL_PREFERRED) {
> +	if (policy->mode == MPOL_PREFERRED ||
> +	    policy->mode == MPOL_PREFERRED_MANY) {
>  		nd = first_node(policy->nodes);
>  	} else {
>  		/*

Do we really want to have the preferred node to be always the first node
in the node mask? Shouldn't that strive for a locality as well? Existing
callers already prefer numa_node_id() - aka local node - and I belive we
shouldn't just throw that away here.

> @@ -1931,6 +1954,7 @@ unsigned int mempolicy_slab_node(void)
>  
>  	switch (policy->mode) {
>  	case MPOL_PREFERRED:
> +	case MPOL_PREFERRED_MANY:
>  		return first_node(policy->nodes);

Similarly here but I am not really familiar with the slab numa code
enough to have strong opinions here.

> @@ -2173,10 +2198,12 @@ struct page *alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
>  		 * node and don't fall back to other nodes, as the cost of
>  		 * remote accesses would likely offset THP benefits.
>  		 *
> -		 * If the policy is interleave, or does not allow the current
> -		 * node in its nodemask, we allocate the standard way.
> +		 * If the policy is interleave or multiple preferred nodes, or
> +		 * does not allow the current node in its nodemask, we allocate
> +		 * the standard way.
>  		 */
> -		if (pol->mode == MPOL_PREFERRED)
> +		if ((pol->mode == MPOL_PREFERRED ||
> +		     pol->mode == MPOL_PREFERRED_MANY))
>  			hpage_node = first_node(pol->nodes);

Same here.

> @@ -2451,6 +2479,9 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
>  		break;
>  
>  	case MPOL_PREFERRED:
> +	case MPOL_PREFERRED_MANY:
> +		if (node_isset(curnid, pol->nodes))
> +			goto out;
>  		polnid = first_node(pol->nodes);
>  		break;

I do not follow what is the point of using first_node here. Either the
node is in the mask or it is misplaced. What are you trying to achieve
here?
-- 
Michal Hocko
SUSE Labs