Re: [PATCH v7 2/2] loop: Better discard support for block devices

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Nov 14, 2019 at 03:50:08PM -0800, Evan Green wrote:
> If the backing device for a loop device is itself a block device,
> then mirror the "write zeroes" capabilities of the underlying
> block device into the loop device. Copy this capability into both
> max_write_zeroes_sectors and max_discard_sectors of the loop device.
> 
> The reason for this is that REQ_OP_DISCARD on a loop device translates
> into blkdev_issue_zeroout(), rather than blkdev_issue_discard(). This
> presents a consistent interface for loop devices (that discarded data
> is zeroed), regardless of the backing device type of the loop device.
> There should be no behavior change for loop devices backed by regular
> files.
> 
> This change fixes blktest block/003, and removes an extraneous
> error print in block/013 when testing on a loop device backed
> by a block device that does not support discard.
> 
> Signed-off-by: Evan Green <evgreen@chromium.org>
> Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
> Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
> ---
> 
> Changes in v7:
> - Rebase on top of Darrick's patch
> - Tweak opening line of commit description (Darrick)
> 
> Changes in v6: None
> Changes in v5:
> - Don't mirror discard if lo_encrypt_key_size is non-zero (Gwendal)
> 
> Changes in v4:
> - Mirror blkdev's write_zeroes into loopdev's discard_sectors.
> 
> Changes in v3:
> - Updated commit description
> 
> Changes in v2: None
> 
>  drivers/block/loop.c | 40 +++++++++++++++++++++++++++++-----------
>  1 file changed, 29 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 6a9fe1f9fe84..e8f23e4b78f7 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -427,11 +427,12 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
>  	 * information.
>  	 */
>  	struct file *file = lo->lo_backing_file;
> +	struct request_queue *q = lo->lo_queue;
>  	int ret;
>  
>  	mode |= FALLOC_FL_KEEP_SIZE;
>  
> -	if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> +	if (!blk_queue_discard(q)) {
>  		ret = -EOPNOTSUPP;
>  		goto out;
>  	}
> @@ -862,6 +863,21 @@ static void loop_config_discard(struct loop_device *lo)
>  	struct file *file = lo->lo_backing_file;
>  	struct inode *inode = file->f_mapping->host;
>  	struct request_queue *q = lo->lo_queue;
> +	struct request_queue *backingq;
> +
> +	/*
> +	 * If the backing device is a block device, mirror its zeroing
> +	 * capability. REQ_OP_DISCARD translates to a zero-out even when backed
> +	 * by block devices to keep consistent behavior with file-backed loop
> +	 * devices.
> +	 */
> +	if (S_ISBLK(inode->i_mode) && !lo->lo_encrypt_key_size) {
> +		backingq = bdev_get_queue(inode->i_bdev);
> +		blk_queue_max_discard_sectors(q,
> +			backingq->limits.max_write_zeroes_sectors);

max_discard_sectors?

--D

> +
> +		blk_queue_max_write_zeroes_sectors(q,
> +			backingq->limits.max_write_zeroes_sectors);
>  
>  	/*
>  	 * We use punch hole to reclaim the free space used by the
> @@ -869,22 +885,24 @@ static void loop_config_discard(struct loop_device *lo)
>  	 * encryption is enabled, because it may give an attacker
>  	 * useful information.
>  	 */
> -	if ((!file->f_op->fallocate) ||
> -	    lo->lo_encrypt_key_size) {
> +	} else if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
>  		q->limits.discard_granularity = 0;
>  		q->limits.discard_alignment = 0;
>  		blk_queue_max_discard_sectors(q, 0);
>  		blk_queue_max_write_zeroes_sectors(q, 0);
> -		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
> -		return;
> -	}
>  
> -	q->limits.discard_granularity = inode->i_sb->s_blocksize;
> -	q->limits.discard_alignment = 0;
> +	} else {
> +		q->limits.discard_granularity = inode->i_sb->s_blocksize;
> +		q->limits.discard_alignment = 0;
>  
> -	blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
> -	blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
> -	blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
> +		blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
> +		blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
> +	}
> +
> +	if (q->limits.max_write_zeroes_sectors)
> +		blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
> +	else
> +		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
>  }
>  
>  static void loop_unprepare_queue(struct loop_device *lo)
> -- 
> 2.21.0
>