[RFC PATCH 1/3] memory tiering: read last_cpupid correctly in do_huge_pmd_numa_page()

[Zi Yan]

From: Zi Yan <ziy@xxxxxxxxxx>

last_cpupid is only available when memory tiering is off or the folio
is in toptier node. Complete the check to read last_cpupid when it is
available.

Before the fix, the default last_cpupid will be used even if memory
tiering mode is turned off at runtime instead of the actual value. This
can prevent task_numa_fault() from getting right numa fault stats, but
should not cause any crash. User might see performance changes after the
fix.

Fixes: 33024536bafd ("memory tiering: hot page selection with hint page fault latency")
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 mm/huge_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d7c84480f1a4..07d9dde4ca33 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1705,7 +1705,8 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
 	 * For memory tiering mode, cpupid of slow memory page is used
 	 * to record page access time.  So use default value.
 	 */
-	if (node_is_toptier(nid))
+	if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) ||
+	    node_is_toptier(nid))
 		last_cpupid = folio_last_cpupid(folio);
 	target_nid = numa_migrate_prep(folio, vmf, haddr, nid, &flags);
 	if (target_nid == NUMA_NO_NODE)
-- 
2.43.0