Re: [PATCH 3/3] mm/numa_balancing:Allow migrate on protnone reference with MPOL_PREFERRED_MANY policy

[Michal Hocko]

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