[PATCH 8/9] mm/z3fold: fix z3fold_reclaim_page races with z3fold_free

[Miaohe Lin]

Think about the below scene:

CPU1				CPU2
z3fold_reclaim_page		z3fold_free
 spin_lock(&pool->lock)		 get_z3fold_header -- hold page_lock
 kref_get_unless_zero
				 kref_put--zhdr->refcount can be 1 now
 !z3fold_page_trylock
  kref_put -- zhdr->refcount is 0 now
   release_z3fold_page
    WARN_ON(!list_empty(&zhdr->buddy)); -- we're on buddy now!
    spin_lock(&pool->lock); -- deadlock here!

z3fold_reclaim_page might race with z3fold_free and will lead to pool lock
deadlock and zhdr buddy non-empty warning. To fix this, defer getting the
refcount until page_lock is held just like what __z3fold_alloc does. Note
this has the side effect that we won't break the reclaim if we meet a soon
to be released z3fold page now.

Fixes: dcf5aedb24f8 ("z3fold: stricter locking and more careful reclaim")
Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
---
 mm/z3fold.c | 18 +++---------------
 1 file changed, 3 insertions(+), 15 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 4a3cd2ff15b0..a7769befd74e 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -519,13 +519,6 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
 	atomic64_dec(&pool->pages_nr);
 }
 
-static void release_z3fold_page(struct kref *ref)
-{
-	struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
-						refcount);
-	__release_z3fold_page(zhdr, false);
-}
-
 static void release_z3fold_page_locked(struct kref *ref)
 {
 	struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
@@ -1317,12 +1310,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 				break;
 			}
 
-			if (kref_get_unless_zero(&zhdr->refcount) == 0) {
-				zhdr = NULL;
-				break;
-			}
 			if (!z3fold_page_trylock(zhdr)) {
-				kref_put(&zhdr->refcount, release_z3fold_page);
 				zhdr = NULL;
 				continue; /* can't evict at this point */
 			}
@@ -1333,14 +1321,14 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 			 */
 			if (zhdr->foreign_handles ||
 			    test_and_set_bit(PAGE_CLAIMED, &page->private)) {
-				if (!kref_put(&zhdr->refcount,
-						release_z3fold_page_locked))
-					z3fold_page_unlock(zhdr);
+				z3fold_page_unlock(zhdr);
 				zhdr = NULL;
 				continue; /* can't evict such page */
 			}
 			list_del_init(&zhdr->buddy);
 			zhdr->cpu = -1;
+			/* See comment in __z3fold_alloc. */
+			kref_get(&zhdr->refcount);
 			break;
 		}
 
-- 
2.23.0