[PATCH] mm/migrate: fix stale partially_mapped arg to deferred_split_folio()

[Deepanshu Kartikey <kartikey406@gmail.com>]

In migrate_folio_move(), src_partially_mapped is sampled from the source
folio before move_to_new_folio() is called:

  if (folio_order(src) > 1 &&
      !data_race(list_empty(&src->_deferred_list))) {
          src_deferred_split = true;
          src_partially_mapped = folio_test_partially_mapped(src);
  }

A concurrent thread can unmap pages from the source folio between this
read and the actual migration, making the sampled value stale.

After move_to_new_folio() succeeds, __folio_migrate_mapping() has already
copied all folio flags including PG_partially_mapped from src to dst.
Passing the stale src_partially_mapped=false to deferred_split_folio(dst)
while dst already has PG_partially_mapped=true triggers the invariant
check in deferred_split_folio():

  VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio)

at mm/huge_memory.c:4371, because the argument contradicts the flag
already set on the folio.

Fix this by removing the src_partially_mapped variable entirely and
reading PG_partially_mapped directly from dst after move_to_new_folio()
completes, where it is authoritative and race-free.

Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue")
Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085
Tested-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 mm/migrate.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 05cb408846f2..11236779e910 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1361,7 +1361,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 	int old_page_state = 0;
 	struct anon_vma *anon_vma = NULL;
 	bool src_deferred_split = false;
-	bool src_partially_mapped = false;
 	struct list_head *prev;
 
 	__migrate_folio_extract(dst, &old_page_state, &anon_vma);
@@ -1378,7 +1377,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 	if (folio_order(src) > 1 &&
 	    !data_race(list_empty(&src->_deferred_list))) {
 		src_deferred_split = true;
-		src_partially_mapped = folio_test_partially_mapped(src);
 	}
 
 	rc = move_to_new_folio(dst, src, mode);
@@ -1404,11 +1402,13 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 	/*
 	 * Requeue the destination folio on the deferred split queue if
 	 * the source was on the queue.  The source is unqueued in
-	 * __folio_migrate_mapping(), so we recorded the state from
-	 * before move_to_new_folio().
+	 * __folio_migrate_mapping(). Read PG_partially_mapped directly from
+	 * dst: move_to_new_folio() copies all flags from src to dst, so dst
+	 * now holds the correct authoritative state. The pre-migration value
+	 * sampled from src is racy and must not be used.
 	 */
 	if (src_deferred_split)
-		deferred_split_folio(dst, src_partially_mapped);
+		deferred_split_folio(dst, folio_test_partially_mapped(dst));
 
 out_unlock_both:
 	folio_unlock(dst);
-- 
2.43.0