Re: [PATCH v2] memory tiering: Do not allow promotion if NUMA_BALANCING_MEMORY_TIERING is disabled

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 23 Mar 2026 04:48:49 -0500 Donet Tom <donettom@linux.ibm.com> wrote:

> In the current implementation, if NUMA_BALANCING_MEMORY_TIERING is
> disabled and the pages are on the lower tier, the pages may still be
> promoted.
> 
> This happens because task_numa_work() updates the last_cpupid field to
> record the last access time only when NUMA_BALANCING_MEMORY_TIERING is
> enabled and the folio is on the lower tier. If
> NUMA_BALANCING_MEMORY_TIERING is disabled, the last_cpupid field
> can retains a valid last CPU id.
> 
> In should_numa_migrate_memory(), the decision checks whether
> NUMA_BALANCING_MEMORY_TIERING is disabled, the folio is on the lower
> tier, and last_cpupid is invalid. However, the last_cpupid can be
> valid when NUMA_BALANCING_MEMORY_TIERING is disabled, the condition
> evaluates to false and migration is allowed.
> 
> This patch prevents promotion when NUMA_BALANCING_MEMORY_TIERING is
> disabled and the folio is on the lower tier.
> 
> Behavior before this change:
> ============================
>   - If NUMA_BALANCING_NORMAL is enabled, migration occurs between
>     nodes within the same memory tier, and promotion from lower
>     tier to higher tier may also happen.
> 
>   - If NUMA_BALANCING_MEMORY_TIERING is enabled, promotion from
>     lower tier to higher tier nodes is allowed.
> 
> Behavior after this change:
> ===========================
>   - If NUMA_BALANCING_NORMAL is enabled, migration will occur only
>     between nodes within the same memory tier.
> 
>   - If NUMA_BALANCING_MEMORY_TIERING is enabled, promotion from lower
>     tier to higher tier nodes will be allowed.
> 
>   - If both NUMA_BALANCING_MEMORY_TIERING and NUMA_BALANCING_NORMAL are
>     enabled, both migration (same tier) and promotion (cross tier) are
>     allowed.

There was no feedback on this, nor on your v1.

> Fixes: 33024536bafd ("memory tiering: hot page selection with hint page fault latency")

Ying Huang seems to have moved around a bit - let me add a couple more
email addresses.  Apologies if we have multiple Ying Huangs!

Rik, Mel?  It's a bugfix.

Thanks.



From: Donet Tom <donettom@linux.ibm.com>
Subject: memory tiering: do not allow promotion if NUMA_BALANCING_MEMORY_TIERING is disabled
Date: Mon, 23 Mar 2026 04:48:49 -0500

In the current implementation, if NUMA_BALANCING_MEMORY_TIERING is
disabled and the pages are on the lower tier, the pages may still be
promoted.

This happens because task_numa_work() updates the last_cpupid field to
record the last access time only when NUMA_BALANCING_MEMORY_TIERING is
enabled and the folio is on the lower tier.  If
NUMA_BALANCING_MEMORY_TIERING is disabled, the last_cpupid field can
retains a valid last CPU id.

In should_numa_migrate_memory(), the decision checks whether
NUMA_BALANCING_MEMORY_TIERING is disabled, the folio is on the lower tier,
and last_cpupid is invalid.  However, the last_cpupid can be valid when
NUMA_BALANCING_MEMORY_TIERING is disabled, the condition evaluates to
false and migration is allowed.

This patch prevents promotion when NUMA_BALANCING_MEMORY_TIERING is
disabled and the folio is on the lower tier.

Behavior before this change:
============================
  - If NUMA_BALANCING_NORMAL is enabled, migration occurs between
    nodes within the same memory tier, and promotion from lower
    tier to higher tier may also happen.

  - If NUMA_BALANCING_MEMORY_TIERING is enabled, promotion from
    lower tier to higher tier nodes is allowed.

Behavior after this change:
===========================
  - If NUMA_BALANCING_NORMAL is enabled, migration will occur only
    between nodes within the same memory tier.

  - If NUMA_BALANCING_MEMORY_TIERING is enabled, promotion from lower
    tier to higher tier nodes will be allowed.

  - If both NUMA_BALANCING_MEMORY_TIERING and NUMA_BALANCING_NORMAL are
    enabled, both migration (same tier) and promotion (cross tier) are
    allowed.

Link: https://lkml.kernel.org/r/20260323094849.3903-1-donettom@linux.ibm.com
Fixes: 33024536bafd ("memory tiering: hot page selection with hint page fault latency")
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Ben Segall <bsegall@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: "Huang, Ying" <huang.ying.caritas@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/sched/fair.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/kernel/sched/fair.c~memory-tiering-do-not-allow-promotion-if-numa_balancing_memory_tiering-is-disabled
+++ a/kernel/sched/fair.c
@@ -2024,8 +2024,12 @@ bool should_numa_migrate_memory(struct t
 	this_cpupid = cpu_pid_to_cpupid(dst_cpu, current->pid);
 	last_cpupid = folio_xchg_last_cpupid(folio, this_cpupid);
 
+	/*
+	 * Do not allow promotion if NUMA_BALANCING_MEMORY_TIERING is disabled
+	 * and the pages are on the lower tier.
+	 */
 	if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) &&
-	    !node_is_toptier(src_nid) && !cpupid_valid(last_cpupid))
+	    !node_is_toptier(src_nid))
 		return false;
 
 	/*
_