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

[Minchan Kim]

On Mon, Aug 05, 2013 at 04:18:34PM +0900, Minchan Kim wrote:
> 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,
>  			       meta->compress_workmem);
>  
> @@ -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);
>  	up_write(&zram->lock);
>  	atomic64_inc(&zram->stats.notify_free);
> -- 
> 1.7.9.5
> 

How about this version?