zram: prevent data loss in error cases of function zram_bvec_write()

In function zram_bvec_write(), previous data at the index is
already freed by function zram_free_page().
When failed to compress or zs_malloc, there is no way to restore old data.
Therefore, free previous data when it's about to update.

Also, no need to check whether table is not empty outside of
function zram_free_page(), because the function properly checks inside.
Signed-off-by: Sunghan Suh <sunghan.suh@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zram/zram_drv.c

@@ -418,14 +418,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 			goto out;
 	}
 
-	/*
-	 * System overwrites unused sectors. Free memory associated
-	 * with this sector now.
-	 */
-	if (meta->table[index].handle ||
-	    zram_test_flag(meta, index, ZRAM_ZERO))
-		zram_free_page(zram, index);
-
 	user_mem = kmap_atomic(page);
 
 	if (is_partial_io(bvec)) {
@@ -439,6 +431,9 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 
 	if (page_zero_filled(uncmem)) {
 		kunmap_atomic(user_mem);
+		/* Free memory associated with this sector now. */
+		zram_free_page(zram, index);
+
 		zram->stats.pages_zero++;
 		zram_set_flag(meta, index, ZRAM_ZERO);
 		ret = 0;
@@ -486,6 +481,12 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 
 	zs_unmap_object(meta->mem_pool, handle);
 
+	/*
+	 * Free memory associated with this sector
+	 * before overwriting unused sectors.
+	 */
+	zram_free_page(zram, index);
+
 	meta->table[index].handle = handle;
 	meta->table[index].size = clen;