mm/ksm: mark migration stores with WRITE_ONCE()

ksm_get_folio() deliberately samples stable_node->kpfn and folio->mapping without taking the folio lock because the KSM folio may be migrated concurrently. folio_migrate_ksm() updates the same state using plain assignments.

The reader can load the old kpfn, then the migrator can store the new kpfn, execute smp_wmb(), and clear the old folio's mapping before the reader checks that mapping. Thus the initial kpfn load can overlap its update and the subsequent mapping load can overlap the clear, with no common lock. This leaves marked READ_ONCE() accesses racing with plain stores.

The kernel reported:

  BUG: KCSAN: data-race in folio_migrate_ksm / ksm_get_folio

read (marked) to 0xffff8ce401421330 of 8 bytes by task 48 on cpu 3:

   ksm_get_folio+0x7f/0x2a0
   ksm_scan_thread+0x1635/0x3330
   kthread+0x1af/0x1f0

write to 0xffff8ce401421330 of 8 bytes by task 102 on cpu 1:

   folio_migrate_ksm+0x6a/0xd0
   folio_migrate_flags+0x193/0x420
   __migrate_folio.isra.0+0x162/0x1a0
   migrate_folio+0x4c/0x70
   move_to_new_folio+0xd6/0x170

Use WRITE_ONCE() for both stores to pair them with the existing lockless reads. This preserves the existing smp_wmb()/smp_rmb() migration protocol and control flow while preventing compiler transformations of the shared accesses.

---
Changes in v2:
- Add the Fixes tag requested during review.
- Add Cc: [email protected].
- Add David Hildenbrand's Acked-by.

 mm/ksm.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

index b4142746777e..bec6fea0fdb4 100644
@@ -1116,7 +1116,8 @@ static inline void folio_set_stable_node(struct folio *folio,
 					 struct ksm_stable_node *stable_node)
 {
 	VM_WARN_ON_FOLIO(folio_test_anon(folio) && PageAnonExclusive(&folio->page), folio);
-	folio->mapping = (void *)((unsigned long)stable_node | FOLIO_MAPPING_KSM);
+	WRITE_ONCE(folio->mapping,
+		   (void *)((unsigned long)stable_node | FOLIO_MAPPING_KSM));
 }
 
 #ifdef CONFIG_SYSFS

@@ -3318,7 +3319,7 @@ void folio_migrate_ksm(struct folio *newfolio, struct folio *folio)
 	stable_node = folio_stable_node(folio);
 	if (stable_node) {
 		VM_BUG_ON_FOLIO(stable_node->kpfn != folio_pfn(folio), folio);
-		stable_node->kpfn = folio_pfn(newfolio);
+		WRITE_ONCE(stable_node->kpfn, folio_pfn(newfolio));
 		/*
 		 * newfolio->mapping was set in advance; now we need smp_wmb()
 		 * to make sure that the new stable_node->kpfn is visible

2.43.0

As usual, AI review looked at the surrounding code and went nuts. Please check?

	https://sashiko.dev/#/patchset/[email protected]

I don't think any of these are truly pertinent to your fix (which I shall apply), but maybe while you're in there and all fired up ;)

Thanks!