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SubjectRe: [PATCH v4 2/2] loop: Better discard support for block devices
On Thu, May 2, 2019 at 10:44 AM Evan Green <evgreen@chromium.org> wrote:
>
> If the backing device for a loop device is 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.
>
> While in there, differentiate between REQ_OP_DISCARD and
> REQ_OP_WRITE_ZEROES, which are different for block devices,
> but which the loop device had just been lumping together, since
> they're largely the same for 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>
> ---
>
> 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 | 57 ++++++++++++++++++++++++++++----------------
> 1 file changed, 37 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index bbf21ebeccd3..ca6983a2c975 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -417,19 +417,14 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq,
> return ret;
> }
>
> -static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
> +static int lo_discard(struct loop_device *lo, struct request *rq,
> + int mode, loff_t pos)
> {
> - /*
> - * We use punch hole to reclaim the free space used by the
> - * image a.k.a. discard. However we do not support discard if
> - * encryption is enabled, because it may give an attacker
> - * useful information.
> - */
> struct file *file = lo->lo_backing_file;
> - int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
> + struct request_queue *q = lo->lo_queue;
> int ret;
>
> - if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> + if (!blk_queue_discard(q)) {
> ret = -EOPNOTSUPP;
> goto out;
> }
> @@ -599,8 +594,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
> case REQ_OP_FLUSH:
> return lo_req_flush(lo, rq);
> case REQ_OP_DISCARD:
> + return lo_discard(lo, rq,
> + FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pos);
> +
> case REQ_OP_WRITE_ZEROES:
> - return lo_discard(lo, rq, pos);
> + return lo_discard(lo, rq,
> + FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE, pos);
> +
> case REQ_OP_WRITE:
> if (lo->transfer)
> return lo_write_transfer(lo, rq, pos);
> @@ -854,6 +854,21 @@ 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)) {

Gwendal pointed out elsewhere that this should be if
(S_ISBLK(inode->i_mode) && (lo->lo_encrypt_key_size == 0)). I think
that's correct because like the file-backed device, we want to fail
discard, forcing the user to manually zero out regions and write out
the encrypted zeroes. I'll plan to send a v5 soon.

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