[PATCH v4 2/7] mm: allow shared folios to be promoted to a fast tier

[Gregory Price <gourry@gourry.net>]

From: "Gregory Price (Meta)" <gourry@gourry.net>

NUMA balancing rejects shared copy-on-write folios and executable
file folios mapped by multiple processes to avoid east-west
migration bouncing. These checks break promotion from slow memory.

Allow such folios to participate when the folio being migrated is
a low-tier folio and the destination is top-tier (south->north).
This allows promotion, but prevents east-west or north->south
migrations (north->south is handled by reclaim demotion).

Keep the existing restrictions for ordinary placement and for
migrations that are not slow-to-top-tier promotions.

Rename folio_use_access_time() to folio_numab_promotable() so the name
captures the combined memory-tiering and source-tier test. The helper
retains its existing behavior.

Fixes: c574bbe91703 ("NUMA balancing: optimize page placement for memory tiering system")
Cc: stable@vger.kernel.org
Suggested-by: Zi Yan <ziy@nvidia.com>
Link: https://lore.kernel.org/r/DLHS4KFPQ86I.1J4LN3352RI71@nvidia.com
Assisted-by: LLM
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 include/linux/mm.h  |  5 +++--
 kernel/sched/fair.c |  2 +-
 mm/memory-tiers.c   |  9 ++++++---
 mm/memory.c         |  2 +-
 mm/mempolicy.c      | 11 ++++++++---
 mm/migrate.c        | 13 +++++++++----
 6 files changed, 28 insertions(+), 14 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 623ae61e52cb..69107e877fd1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2672,7 +2672,7 @@ static inline void vma_set_access_pid_bit(struct vm_area_struct *vma)
 	}
 }
 
-bool folio_use_access_time(struct folio *folio);
+bool folio_numab_promotable(struct folio *folio);
 #else /* !CONFIG_NUMA_BALANCING */
 static inline int folio_xchg_last_cpupid(struct folio *folio, int cpupid)
 {
@@ -2726,7 +2726,8 @@ static inline bool cpupid_match_pid(struct task_struct *task, int cpupid)
 static inline void vma_set_access_pid_bit(struct vm_area_struct *vma)
 {
 }
-static inline bool folio_use_access_time(struct folio *folio)
+
+static inline bool folio_numab_promotable(struct folio *folio)
 {
 	return false;
 }
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c3dfa398d0bd..3be18cf10eca 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2742,7 +2742,7 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
 	 * The pages in slow memory node should be migrated according
 	 * to hot/cold instead of private/shared.
 	 */
-	if (folio_use_access_time(folio)) {
+	if (folio_numab_promotable(folio)) {
 		struct pglist_data *pgdat;
 		unsigned long rate_limit;
 		unsigned int latency, th, def_th;
diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index 25e121851b58..086d5695e656 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -53,16 +53,19 @@ static const struct bus_type memory_tier_subsys = {
 
 #ifdef CONFIG_NUMA_BALANCING
 /**
- * folio_use_access_time - check if a folio reuses cpupid for page access time
+ * folio_numab_promotable - check if NUMA balancing can promote a folio
  * @folio: folio to check
  *
  * folio's _last_cpupid field is repurposed by memory tiering. In memory
  * tiering mode, cpupid of slow memory folio (not toptier memory) is used to
  * record page access time.
  *
- * Return: the folio _last_cpupid is used to record page access time
+ * If memory tiering is disabled, then lowtier has no appreciable meaning,
+ * so we return false (the folio should not be migrated on this distinction).
+ *
+ * Return: true if memory tiering can promote the folio.
  */
-bool folio_use_access_time(struct folio *folio)
+bool folio_numab_promotable(struct folio *folio)
 {
 	return (sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) &&
 	       !node_is_toptier(folio_nid(folio));
diff --git a/mm/memory.c b/mm/memory.c
index 330cde31bf8b..4ab6db22ad4c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6240,7 +6240,7 @@ int numa_migrate_check(struct folio *folio, 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 (folio_use_access_time(folio))
+	if (folio_numab_promotable(folio))
 		*last_cpupid = (-1 & LAST_CPUPID_MASK);
 	else
 		*last_cpupid = folio_last_cpupid(folio);
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 8ff37f60b710..f70f4f795840 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -864,8 +864,13 @@ bool folio_can_map_prot_numa(struct folio *folio, struct vm_area_struct *vma,
 	if (!folio || folio_is_zone_device(folio) || folio_test_ksm(folio))
 		return false;
 
-	/* Also skip shared copy-on-write folios */
-	if (vma_is_cow_mapping(vma) && folio_maybe_mapped_shared(folio))
+	/*
+	 * Shared copy-on-write folios are poor east-west placement candidates.
+	 * When tiering is enabled, folio_numab_promotable() identifies a
+	 * low-tier folio that needs a hint fault for promotion.
+	 */
+	if (vma_is_cow_mapping(vma) && folio_maybe_mapped_shared(folio) &&
+	    !folio_numab_promotable(folio))
 		return false;
 
 	/* Folios are pinned and can't be migrated */
@@ -892,7 +897,7 @@ bool folio_can_map_prot_numa(struct folio *folio, struct vm_area_struct *vma,
 	if (vma_is_single_threaded_private(vma) && nid == numa_node_id())
 		return false;
 
-	if (folio_use_access_time(folio))
+	if (folio_numab_promotable(folio))
 		folio_xchg_access_time(folio, jiffies_to_msecs(jiffies));
 
 	return true;
diff --git a/mm/migrate.c b/mm/migrate.c
index 7bdcdb57652f..c0f5f3ac71fc 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2694,14 +2694,19 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
 
 	if (folio_is_file_lru(folio)) {
 		/*
-		 * Do not migrate file folios that are mapped in multiple
-		 * processes with execute permissions as they are probably
-		 * shared libraries.
+		 * Limit east-west migration of file folios mapped in
+		 * multiple processes with execute permissions as they
+		 * are probably shared libraries (limits bouncing).
+		 *
+		 * If this is a low-tier folio, only migrate if the target
+		 * node is toptier (this allows south->north migration while
+		 * disallowing east-west migration between slow tiers).
 		 *
 		 * See folio_maybe_mapped_shared() on possible imprecision
 		 * when we cannot easily detect if a folio is shared.
 		 */
-		if ((vma->vm_flags & VM_EXEC) && folio_maybe_mapped_shared(folio))
+		if ((vma->vm_flags & VM_EXEC) && folio_maybe_mapped_shared(folio) &&
+		    (!folio_numab_promotable(folio) || !node_is_toptier(node)))
 			return -EACCES;
 
 		/*
-- 
2.55.0