Re: [PATCH v4 2/2] loop: Better discard support for block devices

[Evan Green]

On Thu, May 2, 2019 at 10:44 AM Evan Green <evgreen@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> ---
>
> 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.