Re: [RFC PATCH] dm: add deviceless messages

[Matthew Sakai]

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 <mpato...@redhat.com>

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 <msa...@redhat.com>

---
  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;