[PATCH] zram: bug fix: delay lock holding in zram_slot_free_noity

From: Minchan Kim
Date: Mon Aug 05 2013 - 03:18:06 EST

I was preparing to promote zram and it was almost done.
Before sending patch, I tried to test and eyebrows went up.

[1] introduced down_write in zram_slot_free_notify to prevent race
between zram_slot_free_notify and zram_bvec_[read|write]. The race
could happen if somebody who has right permission to open swap device
is reading swap device while it is used by swap in parallel.

However, zram_slot_free_notify is called with holding spin_lock of
swap layer so we shouldn't avoid holing mutex. Otherwise, lockdep
warns it.

I guess, best solution is to redesign zram lock scheme totally but
we are on the verge of promoting so it's not desirable to change a lot
critical code and such big change isn't good shape for backporting to
stable trees so I think the simple patch is best at the moment.

[1] [57ab0485, zram: use zram->lock to protect zram_free_page()
in swap free notify path]

Cc: Jiang Liu <jiang.liu@xxxxxxxxxx>
Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
drivers/staging/zram/zram_drv.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
index 7ebf91d..7b574c4 100644
--- a/drivers/staging/zram/zram_drv.c
+++ b/drivers/staging/zram/zram_drv.c
@@ -440,6 +440,13 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
goto out;

+ /*
+ * zram_slot_free_notify could miss free so that let's
+ * double check.
+ */
+ if (unlikely(meta->table[index].handle))
+ zram_free_page(zram, index);
ret = lzo1x_1_compress(uncmem, PAGE_SIZE, src, &clen,

@@ -727,7 +734,13 @@ static void zram_slot_free_notify(struct block_device *bdev,
struct zram *zram;

zram = bdev->bd_disk->private_data;
- down_write(&zram->lock);
+ /*
+ * The function is called in atomic context so down_write should
+ * be prohibited. If we couldn't hold a mutex, the free could be
+ * handled by zram_bvec_write later when same index is overwritten.
+ */
+ if (!down_write_trylock(&zram->lock))
+ return;
zram_free_page(zram, index);

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