[PATCH] zram: avoid kunmap_atomic a NULL pointer

[Weijie Yang <weijie.yang@samsung.com>]

zram could kunmap_atomic a NULL pointer in a rare situation:
a zram page become a full-zeroed page after a partial write io.
The current code doesn't handle this case and kunmap_atomic a
NULL porinter, which panic the kernel.

This patch fixes this issue.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
---
 drivers/block/zram/zram_drv.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 2ad0b5b..3920ee4 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -560,7 +560,8 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 	}
 
 	if (page_zero_filled(uncmem)) {
-		kunmap_atomic(user_mem);
+		if (user_mem)
+			kunmap_atomic(user_mem);
 		/* Free memory associated with this sector now. */
 		bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
 		zram_free_page(zram, index);
-- 
1.7.0.4


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