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

["Aneesh Kumar K.V" <aneesh.kumar@kernel.org>]

On 2/20/24 12:42 AM, Michal Hocko wrote:
> 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.

I think the confusion is between MPOL_F_MOF (migrate on fault) vs MPOL_F_MORON( protnone fault/numa fault).

With MPOL_F_MOF alone what we wanted to achieve was to have have mbind() lazy migrate the pages based on policy node
mask. The change was introduced in commit commit b24f53a0bea3 ("mm: mempolicy: Add MPOL_MF_LAZY") and later dropped by
commit 2cafb582173f ("mempolicy: remove confusing MPOL_MF_LAZY dead code"). We still have mpol_misplaced changes
to handle the node selection for MPOL_F_MOF flag (this is dead code IIUC). 

MPOL_F_MORON was added in commit 5606e3877ad8 ("mm: numa: Migrate on reference policy") and with currently upstream only
MPOL_BIND support that flag. With that flag specified and with the changes in the patch mpol_misplaced becomes

	case MPOL_PREFERRED_MANY:
		if (pol->flags & MPOL_F_MORON) {
			if (!mpol_preferred_should_numa_migrate(thisnid, curnid, pol))
				goto out;
			break;
		}

		/*
		 * 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(thisnid, GFP_HIGHUSER),
				gfp_zone(GFP_HIGHUSER),
				&pol->nodes);
		polnid = zone_to_nid(z->zone);
		break;
 ....
...
       }

	/* Migrate the folio towards the node whose CPU is referencing it */
	if (pol->flags & MPOL_F_MORON) {
		polnid = thisnid;

		if (!should_numa_migrate_memory(current, folio, curnid,
						thiscpu))
			goto out;
	}

	if (curnid != polnid)
		ret = polnid;
out:
	mpol_cond_put(pol);

	return ret;
}




ie, if we can do numa migration, we select the currently executing node as the target node otherwise
we end up returning from the function with ret = NUMA_NO_NODE.

-aneesh