[PATCH v2 03/10] zram: use zram->lock to protect zram_free_page() in swap free notify path

[Jerome Marchand]

On 06/04/2013 06:06 PM, Jiang Liu wrote:
> zram_free_page() is protected by down_write(&zram->lock) when called by
> zram_bvec_write(), but there's no such protection when called by
> zram_slot_free_notify(), which may cause wrong states to zram object.
> 
> There are two possible consequences of this issue:
> 1) It may cause invalid memory access if we read from a zram device used
>    as swap device. (Not sure whether it's legal to make read/write
>    requests to a zram device used as swap device.)

It's never a good idea to mess with a swap device in use, but AFAICT
nothing prevents it (save for file permissions and admin sanity).

Jerome

> 2) It may cause some fields (bad_compress, good_compress, pages_stored)
>    in zram->stats wrong if the swap layer makes concurrently call to
>    zram_slot_free_notify().
> 
> So enhance zram_slot_free_notify() to acquire writer lock on zram->lock
> before calling zram_free_page().
> 
> Signed-off-by: Jiang Liu <jiang.liu at huawei.com>
> Cc: stable at vger.kernel.org
> ---
>  drivers/staging/zram/zram_drv.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
> index 5a2f20b..847d207 100644
> --- a/drivers/staging/zram/zram_drv.c
> +++ b/drivers/staging/zram/zram_drv.c
> @@ -582,7 +582,9 @@ static void zram_slot_free_notify(struct block_device *bdev,
>  	struct zram *zram;
>  
>  	zram = bdev->bd_disk->private_data;
> +	down_write(&zram->lock);
>  	zram_free_page(zram, index);
> +	up_write(&zram->lock);
>  	zram_stat64_inc(zram, &zram->stats.notify_free);
>  }
>  
>