linux-kernel - z3fold: fix potential race in z3fold_reclaim

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Message-Id: <20170913162937.bfff21c7d12b12a5f47639fd@gmail.com>
Date:   Wed, 13 Sep 2017 16:29:37 +0200
From:   Vitaly Wool <vitalywool@...il.com>
To:     Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Cc:     Dan Streetman <ddstreet@...e.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleksiy.Avramchenko@...y.com
Subject: z3fold: fix potential race in z3fold_reclaim_page

It is possible that on a (partially) unsuccessful page reclaim,
kref_put() called in z3fold_reclaim_page() does not yield page
release, but the page is released shortly afterwards by another
thread. Then z3fold_reclaim_page() would try to list_add() that
(released) page again which is obviously a bug.

To avoid that, spin_lock() has to be taken earlier, before the
kref_put() call mentioned earlier.

Signed-off-by: Vitaly Wool <vitalywool@...il.com>
---
 mm/z3fold.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 486550df32be..b04fa3ba1bf2 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -875,16 +875,18 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 				goto next;
 		}
 next:
+		spin_lock(&pool->lock);
 		if (test_bit(PAGE_HEADLESS, &page->private)) {
 			if (ret == 0) {
+				spin_unlock(&pool->lock);
 				free_z3fold_page(page);
 				return 0;
 			}
 		} else if (kref_put(&zhdr->refcount, release_z3fold_page)) {
 			atomic64_dec(&pool->pages_nr);
+			spin_unlock(&pool->lock);
 			return 0;
 		}
-		spin_lock(&pool->lock);
 
 		/*
 		 * Add to the beginning of LRU.
-- 
2.11.0