[PATCH] mm: get the folio's refcnt before clear PG_lru in folio_isolate_lru

["zhaoyang.huang" <zhaoyang.huang@xxxxxxxxxx>]

From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>

Bellowing race happens when the caller of folio_isolate_lru rely on the
refcnt of page cache. Moving folio_get ahead of folio_test_clear_lru to
make it more robust.

0. Thread_isolate calls folio_isolate_lru by holding one refcnt of page
   cache and get preempted before folio_get.
       folio_isolate_lru
           VM_BUG_ON(!folio->refcnt)
           if (folio_test_clear_lru(folio))
               <preempted>
               folio_get()
1. Thread_release calls release_pages and meet the scenario of the folio
   get its refcnt of page cache removed before folio_put_testzero
       release_pages
           <folio is removed from page cache>
           folio_put_testzero(folio) == true
           <refcnt added by collection is the only one here and get
deducted>
           if(folio_test_clear_lru(folio))
                 lruvec_del_folio(folio)
           <folio failed to be deleted from LRU>
           list_add(folio, pages_to_free);
           <LRU's integrity is broken by above list_add>

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3ef654addd44..42f15ca06e09 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1731,10 +1731,10 @@ bool folio_isolate_lru(struct folio *folio)
 
 	VM_BUG_ON_FOLIO(!folio_ref_count(folio), folio);
 
+	folio_get(folio);
 	if (folio_test_clear_lru(folio)) {
 		struct lruvec *lruvec;
 
-		folio_get(folio);
 		lruvec = folio_lruvec_lock_irq(folio);
 		lruvec_del_folio(lruvec, folio);
 		unlock_page_lruvec_irq(lruvec);
-- 
2.25.1