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

[Donet Tom <donettom@linux.ibm.com>]

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.

Fixes: 33024536bafd ("memory tiering: hot page selection with hint page fault latency")
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
---
v1 -> v2
========
1. Dropped changes in task_numa_fault() since the original changes
   already handle runtime disabling of NUMA_BALANCING_MEMORY_TIERING.

v1 -> https://lore.kernel.org/all/20260320092251.1290207-1-donettom@linux.ibm.com/
---
 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bf948db905ed..4b43809a3fb1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2024,8 +2024,12 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
 	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;
 
 	/*
-- 
2.47.1