[RFC PATCH v6 1/5] mm: migrate: Allow misplaced migration without VMA

[Bharata B Rao <bharata@amd.com>]

We want isolation of misplaced folios to work in contexts
where VMA isn't available, typically when performing migrations
from a kernel thread context. In order to prepare for that,
allow migrate_misplaced_folio_prepare() to be called with
a NULL VMA.

When migrate_misplaced_folio_prepare() is called with non-NULL
VMA, it will check if the folio is mapped shared and that requires
holding PTL lock. This path isn't taken when the function is
invoked with NULL VMA (migration outside of process context).
Therefore, when VMA == NULL, migrate_misplaced_folio_prepare()
does not require the caller to hold the PTL.

Signed-off-by: Bharata B Rao <bharata@amd.com>
---
 mm/migrate.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 2c3d489ecf51..a15184950e65 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2652,7 +2652,12 @@ static struct folio *alloc_misplaced_dst_folio(struct folio *src,
 
 /*
  * Prepare for calling migrate_misplaced_folio() by isolating the folio if
- * permitted. Must be called with the PTL still held.
+ * permitted. Must be called with the PTL still held if called with a non-NULL
+ * vma.
+ *
+ * When called with a NULL vma (e.g., kernel thread initiated migration),
+ * migrate_misplaced_folio_prepare() will allow shared executable folios
+ * to be migrated.
  */
 int migrate_misplaced_folio_prepare(struct folio *folio,
 		struct vm_area_struct *vma, int node)
@@ -2669,7 +2674,7 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
 		 * 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 && (vma->vm_flags & VM_EXEC) && folio_maybe_mapped_shared(folio))
 			return -EACCES;
 
 		/*
-- 
2.34.1