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@xxxxxxxxxxxx>
> Reviewed-by: Gwendal Grignou <gwendal@xxxxxxxxxxxx>
> Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@xxxxxxx>
> ---
>
> 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
>
