Well, I ran into problems in dumping the contents of the mds_sid array, DTrace does not iterate. To fix this, I decided to use a thread counter in the code for mds_ds_path_to_mds_sid:entry. I then increment it in the mds_ds_path_to_mds_sid:return. This allows me to enumerate the data set names.

#!/usr/sbin/dtrace -s

nfsv4:::spe-i-check_open
{
 	printf("%d (%d) from %s is checking %s",
            (uid_t)arg0, (gid_t)arg1, stringof(arg3), stringof(arg2));
}

nfsv4:::spe-i-policy_eval
{
 	printf("Policy %d %s with error %d from %s",
            (uint_t)arg0, (boolean_t)arg1 ? "matched" : "did not match",
            (int)arg2, stringof(arg3));
}

::nfs41_spe_allocate:entry
{
 	self->addr = (struct netbuf *)arg2;
	self->stripe_count = (count4 *)arg4;
	self->unit_size = (uint32_t *)arg5;
	self->mds_sids = (mds_sid **)arg6;

	self->loaded_sids = 0;
}

::nfs41_spe_allocate:return
/args[1] == 0/
{
 	printf("Policy has %d stripes and %u block size",
            *self->stripe_count, *self->unit_size);
}

::nfs41_spe_allocate:return
/args[1] != 0/
{
 	printf("No matching policy");
}

::mds_ds_path_to_mds_sid:entry
{
 	self->ustring = (utf8string *)arg0;
	self->ss_name = stringof(self->ustring->utf8string_val);
	self->mds_sid = (struct mds_sid *)arg1;
}

::mds_ds_path_to_mds_sid:return
/args[1] == 0/
{
 	ss_name = (char *)alloca(self->ustring->utf8string_len + 1);
	 bcopy(self->ustring->utf8string_val, ss_name,
            self->ustring->utf8string_len);
    	 ss_name[self->ustring->utf8string_len + 1] = '\0';
	printf("mds_sid[%d] = %s", self->loaded_sids++,
            stringof(ss_name));
}

::mds_ds_path_to_mds_sid:return
/args[1] != 0/
{
}

The remaining problem was that my first attempt to dump the utf8string resulted in garbage at the end of the output. Which makes sense, as such strings are not NUL terminated. Here is the original code:

::mds_ds_path_to_mds_sid:entry
{
 	u = (utf8string *)arg0;
  	self->ss_name = stringof(u->utf8string_val);
	self->mds_sid = (struct mds_sid *)arg1;
}

::mds_ds_path_to_mds_sid:return
/args[1] == 0/
{
 	printf("mds_sid[%d] = %s", self->loaded_sids++, self->ss_name);
}

There was an interesting thread of discussion over at: [dtrace-discuss] getting extra characters with printf(copyin(a, b)). I tried the approaches suggested by both Chad and Adam, but no luck. I get on getting:

dtrace: error on enabled probe ID 7 (ID 63585: fbt:nfssrv:mds_ds_path_to_mds_sid:return): invalid address (0xffffff01f6a103f8) in action #2 at DIF offset 56

Which to be fair, I more than expected. The issue is that the copyin and copyinstr work on user land memory and the stuff I want to dump is in the kernel. Where that thread really helped me though was it pointed me to using bcopy(). With that in play, things went well:

  1   3502 nfs41_spe_allocate:spe-i-check_open 0 (0) from 10.1.233.12 is checking /pnfs1/nfs41/pnfs-4-02/2/spek.txt
  1   3501 nfs41_spe_allocate:spe-i-policy_eval Policy 1000 did not match with error 0 from 10.1.233.12
  1   3501 nfs41_spe_allocate:spe-i-policy_eval Policy 2000 matched with error 0 from 10.1.233.12
  1  63585    mds_ds_path_to_mds_sid:return mds_sid[0] = pnfs-17-22:pnfs1/ds1n
  1  63585    mds_ds_path_to_mds_sid:return mds_sid[1] = pnfs-17-22:pnfs1/ds8n
  1  63585    mds_ds_path_to_mds_sid:return mds_sid[2] = pnfs-17-22:pnfs2/ds2n
  1  63585    mds_ds_path_to_mds_sid:return mds_sid[3] = pnfs-17-23:pnfs1/ds3n
  1  63585    mds_ds_path_to_mds_sid:return mds_sid[4] = pnfs-17-23:pnfs1/ds7n
  1  63585    mds_ds_path_to_mds_sid:return mds_sid[5] = pnfs-17-23:pnfs2/ds4f
  1  63585    mds_ds_path_to_mds_sid:return mds_sid[6] = pnfs-4-01:pnfs1/d10n
  1  63585    mds_ds_path_to_mds_sid:return mds_sid[7] = pnfs-4-01:pnfs1/ds4
  1  62825        nfs41_spe_allocate:return Policy has 8 stripes and 1000 block size