Re: [PATCH RFC 2/8] dm: Add support for block provisioning

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Sep 15 2022 at 12:48P -0400,
Sarthak Kukreti <sarthakkukreti@chromium.org> wrote:

> From: Sarthak Kukreti <sarthakkukreti@chromium.org>
> 
> Add support to dm devices for REQ_OP_PROVISION. The default mode
> is to pass through the request and dm-thin will utilize it to provision
> blocks.
> 
> Signed-off-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
> ---
>  drivers/md/dm-crypt.c         |  4 +-
>  drivers/md/dm-linear.c        |  1 +
>  drivers/md/dm-table.c         | 17 +++++++
>  drivers/md/dm-thin.c          | 86 +++++++++++++++++++++++++++++++++--
>  drivers/md/dm.c               |  4 ++
>  include/linux/device-mapper.h |  6 +++
>  6 files changed, 113 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> index 159c6806c19b..357f0899cfb6 100644
> --- a/drivers/md/dm-crypt.c
> +++ b/drivers/md/dm-crypt.c
> @@ -3081,6 +3081,8 @@ static int crypt_ctr_optional(struct dm_target *ti, unsigned int argc, char **ar
>  	if (ret)
>  		return ret;
>  
> +	ti->num_provision_bios = 1;
> +
>  	while (opt_params--) {
>  		opt_string = dm_shift_arg(&as);
>  		if (!opt_string) {
> @@ -3384,7 +3386,7 @@ static int crypt_map(struct dm_target *ti, struct bio *bio)
>  	 * - for REQ_OP_DISCARD caller must use flush if IO ordering matters
>  	 */
>  	if (unlikely(bio->bi_opf & REQ_PREFLUSH ||
> -	    bio_op(bio) == REQ_OP_DISCARD)) {
> +	    bio_op(bio) == REQ_OP_DISCARD || bio_op(bio) == REQ_OP_PROVISION)) {
>  		bio_set_dev(bio, cc->dev->bdev);
>  		if (bio_sectors(bio))
>  			bio->bi_iter.bi_sector = cc->start +
> diff --git a/drivers/md/dm-linear.c b/drivers/md/dm-linear.c
> index 3212ef6aa81b..1aa782149428 100644
> --- a/drivers/md/dm-linear.c
> +++ b/drivers/md/dm-linear.c
> @@ -61,6 +61,7 @@ static int linear_ctr(struct dm_target *ti, unsigned int argc, char **argv)
>  	ti->num_discard_bios = 1;
>  	ti->num_secure_erase_bios = 1;
>  	ti->num_write_zeroes_bios = 1;
> +	ti->num_provision_bios = 1;
>  	ti->private = lc;
>  	return 0;
>  
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 332f96b58252..b7f9cb66b7ba 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -1853,6 +1853,18 @@ static bool dm_table_supports_write_zeroes(struct dm_table *t)
>  	return true;
>  }
>  
> +static bool dm_table_supports_provision(struct dm_table *t)
> +{
> +	for (unsigned int i = 0; i < t->num_targets; i++) {
> +		struct dm_target *ti = dm_table_get_target(t, i);
> +
> +		if (ti->num_provision_bios)
> +			return true;
> +	}
> +
> +	return false;
> +}
> +

This needs to go a step further and verify a device in the stack
actually services REQ_OP_PROVISION.

Please see dm_table_supports_discards(): it iterates all devices in
the table and checks that support is advertised.

For discard, DM requires that _all_ devices in a table advertise
support (that is pretty strict and likely could be relaxed to _any_).

You'll need ti->provision_supported (like ->discards_supported) to
advertise actual support is provided by dm-thinp (even if underlying
devices don't support it).

And yeah, dm-thinp passdown support for REQ_OP_PROVISION can follow
later as needed (if there actual HW that would benefit from
REQ_OP_PROVISION).

Mike