This changes the ABI on 64-bit architectures, since an int is only 32-bits there, but a pointer is 64 so the sizeof md_ioctl grows by 4 bytes.

I think there is alignment in play, the struct is not packed.

I believe the rule for manpages is that sentences start on a new line. Also, "can be then" should be "can then be". Finally, the second sentence is a fragment. I suggest this instead:

.It Fl L Ar label
Associate a label (arbitrary string) with the new memory disk.
The label can then be inspected with the
.Nm
.Fl l v
command.

I think you want copystr().

That's right. Use textproc/igor to check the man page for common errors.

I would prefer "can then be instpected with
.Nm
.Fl l v ."
to eliminate the not-really-needed "the" and "command".

Yeah, that's probably better way to phrase it.

Are you sure? Reading copystr(8):

DESCRIPTION
     The copy functions are designed to copy contiguous data from one address
     to another.  All but copystr() copy data from user-space to kernel-space
     or vice-versa.
...
     The copystr() function copies a NUL-terminated string, at most len bytes
     long, from kernel-space address kfaddr to kernel-space address kdaddr.
     The number of bytes actually copied, including the terminating NUL, is
     returned in *done (if done is non-NULL).

I've verified again, the sizeof(struct md_ioctl) remains 448 bytes before and after change on x64. I don't have access to any other supported 64-bit arches to verify.

You're right. I need new glasses. :-p

I feel like this example command should be separated from the rest of the sentence. Maybe:

The label can then be inspected with
.Bd -literal -offset indent
.Nm
.Fl l v
.Ed

(Might need a .Pp after that.)

Done in the latest revision. I have had to merge Nm + Fl... into a single line, though otherwise it would wrap it up separating -l -v to the next line.