On Tue, 12 May 2026, 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]>
This is example code how to use deviceless messages. You can call it with
"dmsetup message linear 0 %Hello"
Mikulas
---
drivers/md/dm-linear.c | 10 ++++++++++
1 file changed, 10 insertions(+)
Index: linux-2.6/drivers/md/dm-linear.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-linear.c 2026-05-11 13:00:08.000000000
+0200
+++ linux-2.6/drivers/md/dm-linear.c 2026-05-11 13:00:14.000000000 +0200
@@ -199,6 +199,15 @@ static size_t linear_dax_recovery_write(
#define linear_dax_recovery_write NULL
#endif
+static int linear_deviceless_message(unsigned int argc, char **argv, char
*result, unsigned int maxlen)
+{
+ if (argc == 1 && !strcasecmp(argv[0], "%Hello")) {
+ snprintf(result, maxlen, "World!");
+ return 1;
+ }
+ return -EINVAL;
+}
+
static struct target_type linear_target = {
.name = "linear",
.version = {1, 5, 0},
@@ -211,6 +220,7 @@ static struct target_type linear_target
.dtr = linear_dtr,
.map = linear_map,
.status = linear_status,
+ .deviceless_message = linear_deviceless_message,
.prepare_ioctl = linear_prepare_ioctl,
.iterate_devices = linear_iterate_devices,
.direct_access = linear_dax_direct_access,
