Re: [dm-devel] [PATCH] dm-zero: support discards

[Milan Broz <gmazyland@gmail.com>]

On 3/6/23 20:10, Mikulas Patocka wrote:
> This patch adds discard support to dm-zero. The discards are ignored.
> It is useful when the user combines dm-zero with other discard-supporting
> targets in the same table; without dm-zero support, discards would be
> disabled for the whole combined device.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Tested-by: Milan Broz <gmazyland@gmail.com>

Just for the context: cryptsetup use dm-zero to mask certain areas
(in Bitlocker-compatible mapping) and discard flag disappeared in this situation
(discard is not supported configuration here, but it should work :)
Originally reported here https://gitlab.com/cryptsetup/cryptsetup/-/issues/718

Propagating discard support in dm-zero is definitely useful for other uses, though.

Thanks,
Milan

> 
> ---
>   drivers/md/dm-table.c |    9 ++++++++-
>   drivers/md/dm-zero.c  |    4 +++-
>   2 files changed, 11 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/drivers/md/dm-zero.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-zero.c
> +++ linux-2.6/drivers/md/dm-zero.c
> @@ -27,6 +27,7 @@ static int zero_ctr(struct dm_target *ti
>   	 * Silently drop discards, avoiding -EOPNOTSUPP.
>   	 */
>   	ti->num_discard_bios = 1;
> +	ti->discards_supported = true;
>   
>   	return 0;
>   }
> @@ -45,6 +46,7 @@ static int zero_map(struct dm_target *ti
>   		zero_fill_bio(bio);
>   		break;
>   	case REQ_OP_WRITE:
> +	case REQ_OP_DISCARD:
>   		/* writes get silently dropped */
>   		break;
>   	default:
> @@ -59,7 +61,7 @@ static int zero_map(struct dm_target *ti
>   
>   static struct target_type zero_target = {
>   	.name   = "zero",
> -	.version = {1, 1, 0},
> +	.version = {1, 2, 0},
>   	.features = DM_TARGET_NOWAIT,
>   	.module = THIS_MODULE,
>   	.ctr    = zero_ctr,
> Index: linux-2.6/drivers/md/dm-table.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-table.c
> +++ linux-2.6/drivers/md/dm-table.c
> @@ -1670,8 +1670,15 @@ int dm_calculate_queue_limits(struct dm_
>   
>   		blk_set_stacking_limits(&ti_limits);
>   
> -		if (!ti->type->iterate_devices)
> +		if (!ti->type->iterate_devices) {
> +			if (ti->discards_supported) {
> +				ti_limits.max_discard_sectors = UINT_MAX;
> +				ti_limits.max_hw_discard_sectors = UINT_MAX;
> +				ti_limits.discard_granularity = 512;
> +				ti_limits.discard_alignment = 0;
> +			}
>   			goto combine_limits;
> +		}
>   
>   		/*
>   		 * Combine queue limits of all the devices this target uses.
> 

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