linux-kernel - Re: [PATCH v4 10/13] mm/mempolicy: VMA allocation for many preferred

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Date:   Wed, 14 Apr 2021 15:14:31 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v4 10/13] mm/mempolicy: VMA allocation for many preferred

On Wed 17-03-21 11:40:07, Feng Tang wrote:
[...]
> @@ -2301,10 +2300,26 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
>  		 * does not allow the current node in its nodemask, we allocate
>  		 * the standard way.
>  		 */
> -		if ((pol->mode == MPOL_PREFERRED ||
> -		     pol->mode == MPOL_PREFERRED_MANY) &&
> -		    !(pol->flags & MPOL_F_LOCAL))
> +		if (pol->mode == MPOL_PREFERRED || !(pol->flags & MPOL_F_LOCAL)) {
>  			hpage_node = first_node(pol->nodes);
> +		} else if (pol->mode == MPOL_PREFERRED_MANY) {
> +			struct zoneref *z;
> +
> +			/*
> +			 * In this policy, with direct reclaim, the normal
> +			 * policy based allocation will do the right thing - try
> +			 * twice using the preferred nodes first, and all nodes
> +			 * second.
> +			 */
> +			if (gfp & __GFP_DIRECT_RECLAIM) {
> +				page = alloc_pages_policy(pol, gfp, order, NUMA_NO_NODE);
> +				goto out;
> +			}
> +
> +			z = first_zones_zonelist(node_zonelist(numa_node_id(), GFP_HIGHUSER),
> +						 gfp_zone(GFP_HIGHUSER), &pol->nodes);
> +			hpage_node = zone_to_nid(z->zone);
> +		}
>  
>  		nmask = policy_nodemask(gfp, pol);
>  		if (!nmask || node_isset(hpage_node, *nmask)) {
> @@ -2330,9 +2345,7 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
>  		}
>  	}
>  
> -	nmask = policy_nodemask(gfp, pol);
> -	preferred_nid = policy_node(gfp, pol, node);
> -	page = __alloc_pages_nodemask(gfp, order, preferred_nid, nmask);
> +	page = alloc_pages_policy(pol, gfp, order, NUMA_NO_NODE);
>  	mpol_cond_put(pol);
>  out:
>  	return page;

OK, it took me a while to grasp this but the code is a mess I have to
say. Not that it was an act of beauty before but this just makes it much
harder to follow. And alloc_pages_policy doesn't really help I have to
say. I would have expected that a dedicated alloc_pages_preferred and a
general fallback to __alloc_pages_nodemask would have been much easier
to follow.
-- 
Michal Hocko
SUSE Labs