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

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Date: Tue, 20 Feb 2024 09:48:57 +0100
From: Michal Hocko <mhocko@...e.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
Cc: Donet Tom <donettom@...ux.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.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 Tue 20-02-24 09:27:25, Aneesh Kumar K.V wrote:
[...]
> 	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;
> }

Ohh, right this code is confusing as hell. Thanks for the clarification.
With this in mind. There should be a comment warning about MPOL_F_MOF
always being unset as the userspace cannot really set it up.

Thanks!

-- 
Michal Hocko
SUSE Labs