[PATCH] mm: zswap: fix missing folio cleanup in writeback race path

[Yosry Ahmed]

In zswap_writeback_entry(), after we get a folio from
__read_swap_cache_async(), we grab the tree lock again to check that the
swap entry was not invalidated and recycled. If it was, we delete the
folio we just added to the swap cache and exit.

However, __read_swap_cache_async() returns the folio locked when it is
newly allocated, which is always true for this path, and the folio is
ref'd. Make sure to unlock and put the folio before returning.

This was discovered by code inspection, probably because this path
handles a race condition that should not happen often, and the bug would
not crash the system, it will only strand the folio indefinitely.

Fixes: 04fc7816089c ("mm: fix zswap writeback race condition")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 mm/zswap.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/zswap.c b/mm/zswap.c
index 8f4a7efc2bdae..00e90b9b5417d 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1448,6 +1448,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	if (zswap_rb_search(&tree->rbroot, swp_offset(entry->swpentry)) != entry) {
 		spin_unlock(&tree->lock);
 		delete_from_swap_cache(folio);
+		folio_unlock(folio);
+		folio_put(folio);
 		return -ENOMEM;
 	}
 	spin_unlock(&tree->lock);
-- 
2.43.0.429.g432eaa2c6b-goog