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.

Link: http://lkml.kernel.org/r/20170913162937.bfff21c7d12b12a5f47639fd@gmail.com
Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: <Oleksiy.Avramchenko@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 486550df32be8589eea953d739ea30e62876b702..b04fa3ba1bf2697223365c68ef98b86319126b0e 100644 (file)
--- 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.