[PATCH v2 2/4] mm: allow shared folios to be promoted to a fast tier

[Gregory Price]

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

NUMA balancing rejects shared copy-on-write folios and executable
file folios mapped by multiple processes to avoid placement bouncing.
These checks also block promotion from slow memory.

Allow such folios to participate when moving from a slow tier to a fast
tier. Keep the existing restrictions for ordinary placement.

Fixes: c574bbe91703 ("NUMA balancing: optimize page placement for memory tiering system")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
---
 mm/mempolicy.c | 8 ++++++--
 mm/migrate.c   | 6 ++++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index a082ccfa09ec..19b599bc2dd1 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -863,8 +863,12 @@ 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 NUMA placement candidates, but
+	 * a hot folio on a slow tier still needs a hint fault for promotion.
+	 */
+	if (vma_is_cow_mapping(vma) && folio_maybe_mapped_shared(folio) &&
+	    !folio_use_access_time(folio))
 		return false;
 
 	/* Folios are pinned and can't be migrated */
diff --git a/mm/migrate.c b/mm/migrate.c
index a369d0c95c38..afd9c97d2389 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2697,12 +2697,14 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
 		/*
 		 * Do not migrate file folios that are mapped in multiple
 		 * processes with execute permissions as they are probably
-		 * shared libraries.
+		 * shared libraries, unless this is a promotion from a slow tier.
 		 *
 		 * 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_use_access_time(folio) || !node_is_toptier(node)))
 			return -EACCES;
 
 		/*
-- 
2.55.0