[PATCH] mm/ksm: mark migration stores with WRITE_ONCE()

[Chengfeng Ye]

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.

Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
---
 mm/ksm.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index b4142746777e..bec6fea0fdb4 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -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