linux-kernel - Re: [PATCH 3/3] mm/numa_balancing:Allow migrate on protnone reference with MPOL_PREFERRED

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Date: Mon, 19 Feb 2024 20:12:23 +0100
From: Michal Hocko <mhocko@...e.com>
To: Donet Tom <donettom@...ux.ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Aneesh Kumar <aneesh.kumar@...nel.org>,
	Huang Ying <ying.huang@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Ben Widawsky <ben.widawsky@...el.com>,
	Feng Tang <feng.tang@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...riel.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Matthew Wilcox <willy@...radead.org>,
	Mike Kravetz <mike.kravetz@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Dan Williams <dan.j.williams@...el.com>,
	Hugh Dickins <hughd@...gle.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH 3/3] mm/numa_balancing:Allow migrate on protnone
 reference with MPOL_PREFERRED_MANY policy

On Mon 19-02-24 20:37:17, Donet Tom wrote:
> 
> On 2/19/24 19:50, Michal Hocko wrote:
> > 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?
> 
> With this patch it will take full nodemask and find out the correct migration target. It will not collapse into first node.

Correct me if I am wrong, but mpol_misplaced will return the first node
of the preffered node mask and then migrate_misplaced_folio would use
it as a target node for alloc_misplaced_dst_folio which performs
__GFP_THISNODE allocation so it won't fall back to a different node.
-- 
Michal Hocko
SUSE Labs