Re: [RFC PATCH] dm: add deviceless messages

On 5/12/26 9:36 AM, Mikulas Patocka wrote:
> The VDO code needs to perform some calculations before creating a device.
> The calculations should be performed in the kernel, so that the same logic
> is not duplicated in the kernel and in the userspace.
> 
> However, currently it is not possible to call a device mapper target
> when no device exists.
> 
> This commit solves the problem by adding "deviceless" messages. If the
> message starts with '%', the name is a name of a target, not a name of a
> device. The message will be directed to the target by calling the
> "deviceless_message" method. The target can perform the requested
> calculations in this method and return the result back to userspace.
> 
> Signed-off-by: Mikulas Patocka <[email protected]>

I do wish that we could clean up the message interface. The sector 
number argument is completely meaningless for a deviceless message, so 
in theory we could distinguish this case by omitting the argument, e.g. 
"dmsetup message <target_type> <command>" and not require the special 
'%' notation, either. But that looks like it would require a lot more 
rearrangement of the message handling code.

I agree this does what we need, so...

Reviewed-by: Matthew Sakai <[email protected]>

> ---
>   drivers/md/dm-ioctl.c         |   57 +++++++++++++++++++++++++++++++++---------
>   include/linux/device-mapper.h |    4 ++
>   2 files changed, 49 insertions(+), 12 deletions(-)
> 
> Index: linux-2.6/drivers/md/dm-ioctl.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-ioctl.c	2026-05-06 22:15:36.000000000 +0200
> +++ linux-2.6/drivers/md/dm-ioctl.c	2026-05-11 12:53:37.000000000 +0200
> @@ -1743,6 +1743,33 @@ static int table_status(struct file *fil
>   	return 0;
>   }
>   
> +static int message_for_module(const char *target_name, unsigned int argc, char **argv,
> +			      char *result, unsigned int maxlen)
> +{
> +	struct target_type *tt;
> +	int r;
> +
> +	if (**argv != '%')
> +		return 2;
> +
> +	tt = dm_get_target_type(target_name);
> +	if (!tt) {
> +		DMERR("unknown target name %s", target_name);
> +		return -EINVAL;
> +	}
> +
> +	if (!tt->deviceless_message) {
> +		DMERR("target %s doesn't support deviceless messages", target_name);
> +		r = -EINVAL;
> +	} else {
> +		r = tt->deviceless_message(argc, argv, result, maxlen);
> +	}
> +
> +	dm_put_target_type(tt);
> +
> +	return r;
> +}
> +
>   /*
>    * Process device-mapper dependent messages.  Messages prefixed with '@'
>    * are processed by the DM core.  All others are delivered to the target.
> @@ -1788,10 +1815,6 @@ static int target_message(struct file *f
>   	char *result = get_result_buffer(param, param_size, &maxlen);
>   	int srcu_idx;
>   
> -	md = find_device(param);
> -	if (!md)
> -		return -ENXIO;
> -
>   	if (tmsg < (struct dm_target_msg *) param->data ||
>   	    invalid_str(tmsg->message, (void *) param + param_size)) {
>   		DMERR("Invalid target message parameters.");
> @@ -1811,9 +1834,19 @@ static int target_message(struct file *f
>   		goto out_argv;
>   	}
>   
> -	r = message_for_md(md, argc, argv, result, maxlen);
> +	r = message_for_module(param->name, argc, argv, result, maxlen);
>   	if (r <= 1)
> +		goto out_deviceless;
> +
> +	md = find_device(param);
> +	if (!md) {
> +		r = -ENXIO;
>   		goto out_argv;
> +	}
> +
> +	r = message_for_md(md, argc, argv, result, maxlen);
> +	if (r <= 1)
> +		goto out_md;
>   
>   	table = dm_get_live_table(md, &srcu_idx);
>   	if (!table) {
> @@ -1838,14 +1871,13 @@ static int target_message(struct file *f
>   		r = -EINVAL;
>   	}
>   
> - out_table:
> +out_table:
>   	dm_put_live_table(md, srcu_idx);
> - out_argv:
> -	kfree(argv);
> - out:
> +out_md:
>   	if (r >= 0)
>   		__dev_status(md, param);
> -
> +	dm_put(md);
> +out_deviceless:
>   	if (r == 1) {
>   		param->flags |= DM_DATA_OUT_FLAG;
>   		if (dm_message_test_buffer_overflow(result, maxlen))
> @@ -1854,8 +1886,9 @@ static int target_message(struct file *f
>   			param->data_size = param->data_start + strlen(result) + 1;
>   		r = 0;
>   	}
> -
> -	dm_put(md);
> +out_argv:
> +	kfree(argv);
> +out:
>   	return r;
>   }
>   
> Index: linux-2.6/include/linux/device-mapper.h
> ===================================================================
> --- linux-2.6.orig/include/linux/device-mapper.h	2026-04-27 19:06:34.000000000 +0200
> +++ linux-2.6/include/linux/device-mapper.h	2026-05-11 12:45:45.000000000 +0200
> @@ -93,6 +93,9 @@ typedef void (*dm_status_fn) (struct dm_
>   typedef int (*dm_message_fn) (struct dm_target *ti, unsigned int argc, char **argv,
>   			      char *result, unsigned int maxlen);
>   
> +typedef int (*dm_deviceless_message_fn) (unsigned int argc, char **argv,
> +			      char *result, unsigned int maxlen);
> +
>   /*
>    * Called with *forward == true. If it remains true, the ioctl should be
>    * forwarded to bdev. If it is reset to false, the target already fully handled
> @@ -214,6 +217,7 @@ struct target_type {
>   	dm_resume_fn resume;
>   	dm_status_fn status;
>   	dm_message_fn message;
> +	dm_deviceless_message_fn deviceless_message;
>   	dm_prepare_ioctl_fn prepare_ioctl;
>   	dm_report_zones_fn report_zones;
>   	dm_busy_fn busy;
>