Message-ID: <aUfi9gn5HS4u4ShU@gourry-fedora-PF4VCD3F>
Date: Sun, 21 Dec 2025 07:07:18 -0500
From: Gregory Price <gourry@...rry.net>
To: Bing Jiao <bingjiao@...gle.com>
Cc: linux-mm@...ck.org, Waiman Long <longman@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>, Tejun Heo <tj@...nel.org>,
	Michal Koutný <mkoutny@...e.com>,
	Qi Zheng <zhengqi.arch@...edance.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Yuanchu Xie <yuanchu@...gle.com>, Wei Xu <weixugc@...gle.com>,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmscan: respect mems_effective in demote_folio_list()


I think this patch can be done without as many changes as proposed here.

> -bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid);
> +void mem_cgroup_node_allowed(struct mem_cgroup *memcg, nodemask_t *nodes);

> -static inline bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid)
> +static inline void mem_cgroup_node_allowed(struct mem_cgroup *memcg,

> -int next_demotion_node(int node);
> +int next_demotion_node(int node, nodemask_t *mask);

> -bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
> +void cpuset_node_allowed(struct cgroup *cgroup, nodemask_t *nodes)

These are some fairly major contract changes, and the names don't make
much sense as a result.

Would be better to just make something like

/* Filter the given nmask based on cpuset.mems.allowed */
mem_cgroup_filter_mems_allowed(memg, nmask);

(or some other, better name)

separate of the existing interfaces, and operate on one scratch-mask if
possible.

> +static int get_demotion_targets(nodemask_t *targets, struct pglist_data *pgdat,
> +				struct mem_cgroup *memcg)
> +{
> +	nodemask_t allowed_mask;
> +	nodemask_t preferred_mask;
> +	int preferred_node;
> +
> +	if (!pgdat)
> +		return NUMA_NO_NODE;
> +
> +	preferred_node = next_demotion_node(pgdat->node_id, &preferred_mask);
> +	if (preferred_node == NUMA_NO_NODE)
> +		return NUMA_NO_NODE;
> +
> +	node_get_allowed_targets(pgdat, &allowed_mask);
> +	mem_cgroup_node_allowed(memcg, &allowed_mask);
> +	if (nodes_empty(allowed_mask))
> +		return NUMA_NO_NODE;
> +
> +	if (targets)
> +		nodes_copy(*targets, allowed_mask);
> +
> +	do {
> +		if (node_isset(preferred_node, allowed_mask))
> +			return preferred_node;
> +
> +		nodes_and(preferred_mask, preferred_mask, allowed_mask);
> +		if (!nodes_empty(preferred_mask))
> +			return node_random(&preferred_mask);
> +
> +		/*
> +		 * Hop to the next tier of preferred nodes. Even if
> +		 * preferred_node is not set in allowed_mask, still can use it
> +		 * to query the nest-best demotion nodes.
> +		 */
> +		preferred_node = next_demotion_node(preferred_node,
> +						    &preferred_mask);
> +	} while (preferred_node != NUMA_NO_NODE);
> +

What you're implementing here is effectively a new feature - allowing
demotion to jump nodes rather than just target the next demotion node.

This is nice, but it should be a separate patch proposal (I think Andrew
said something as much already) - not as part of a fix.

> +	/*
> +	 * Should not reach here, as a non-empty allowed_mask ensures
> +	 * there must have a target node for demotion.

Does it? What if preferred_node is online when calling
next_demotion_node(), but then is offline when
node_get_allowed_targets() is called?


> +	 * Otherwise, it suggests something wrong in node_demotion[]->preferred,
> +	 * where the same-tier nodes have different preferred targets.
> +	 * E.g., if node 0 identifies both nodes 2 and 3 as preferred targets,
> +	 * but nodes 2 and 3 themselves have different preferred nodes.
> +	 */
> +	WARN_ON_ONCE(1);
> +	return node_random(&allowed_mask);

Just returning a random allowed node seems like an objectively poor
result and we should just not demote if we reach this condition. It
likesly means hotplug was happening and node states changed.

> @@ -1041,10 +1090,10 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
>  	if (list_empty(demote_folios))
>  		return 0;
> 
> +	target_nid = get_demotion_targets(&allowed_mask, pgdat, memcg);
>  	if (target_nid == NUMA_NO_NODE)
>  		return 0;
> -
> -	node_get_allowed_targets(pgdat, &allowed_mask);

in the immediate fixup patch, it seems more expedient to just add the
function i described above

/* Filter the given nmask based on cpuset.mems.allowed */
mem_cgroup_filter_mems_allowed(memg, nmask);

and then add that immediate after the node_get_allowed_targets() call.

Then come back around afterwards to add the tier/node-skip functionality
from above in a separate feature patch.

~Gregory

---

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 670fe9fae5ba..1971a8d9475b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1046,6 +1046,11 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
 
        node_get_allowed_targets(pgdat, &allowed_mask);
 
+       /* Filter based on mems_allowed, fail if the result is empty */
+       mem_cgroup_filter_nodemask(memcg, &allowed_mask);
+       if (nodes_empty(allowed_mask))
+               return 0;
+
        /* Demotion ignores all cpuset and mempolicy settings */
        migrate_pages(demote_folios, alloc_demote_folio, NULL,
                      (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,

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