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Date: Mon, 19 Feb 2024 15:20:08 +0100
From: Michal Hocko <mhocko@suse.com>
To: Donet Tom <donettom@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org,
	Aneesh Kumar <aneesh.kumar@kernel.org>,
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	Johannes Weiner <hannes@cmpxchg.org>,
	Matthew Wilcox <willy@infradead.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Vlastimil Babka <vbabka@suse.cz>,
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	Hugh Dickins <hughd@google.com>,
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Subject: Re: [PATCH 3/3] mm/numa_balancing:Allow migrate on protnone
 reference with MPOL_PREFERRED_MANY policy
Message-ID: <ZdNjmOlkMqEOGci1@tiehlicka>
References: <9c3f7b743477560d1c5b12b8c111a584a2cc92ee.1708097962.git.donettom@linux.ibm.com>
 <8d7737208bd24e754dc7a538a3f7f02de84f1f72.1708097962.git.donettom@linux.ibm.com>
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On Sat 17-02-24 01:31:35, Donet Tom wrote:
[...]
> +static inline bool mpol_preferred_should_numa_migrate(int exec_node, int folio_node,
> +					    struct mempolicy *pol)
> +{
> +	/* if the executing node is in the policy node mask, migrate */
> +	if (node_isset(exec_node, pol->nodes))
> +		return true;
> +
> +	/* If the folio node is in policy node mask, don't migrate */
> +	if (node_isset(folio_node, pol->nodes))
> +		return false;
> +	/*
> +	 * both the folio node and executing node are outside the policy nodemask,
> +	 * migrate as normal numa fault migration.
> +	 */
> +	return true;
> +}

I have looked at this again and only now noticed that this doesn't
really work as one would expected. 

        case MPOL_PREFERRED_MANY:
                /*
                 * use current page if in policy nodemask,
                 * else select nearest allowed node, if any.
                 * If no allowed nodes, use current [!misplaced].
                 */
                if (node_isset(curnid, pol->nodes))
                        goto out;
                z = first_zones_zonelist(
                                node_zonelist(numa_node_id(), GFP_HIGHUSER),
                                gfp_zone(GFP_HIGHUSER),
                                &pol->nodes);
                polnid = zone_to_nid(z->zone);
                break;

Will collapse the whole MPOL_PREFERRED_MANY nodemask into the first
notde into that mask. Is that really what we want here? Shouldn't we use
the full nodemask as the migration target? 

-- 
Michal Hocko
SUSE Labs