[RFC PATCH] multipathd: use 0 for initial pg if nr_priority_groups=0 (was: Re: [PATCH 2/2] dm mpath: allow table load with 0 priority groups)

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Jan 31 2011 at  1:09pm -0500,
Moger, Babu <Babu.Moger@lsi.com> wrote:

> Looks good to me. 
> 
> > -----Original Message-----
> > From: Mike Snitzer [mailto:snitzer@redhat.com]
> > Sent: Monday, January 31, 2011 10:14 AM
> > To: dm-devel@redhat.com
> > Cc: Mike Snitzer; Moger, Babu
> > Subject: [PATCH 2/2] dm mpath: allow table load with 0 priority groups
> > 
> > If an mpath device is held open when all paths in the last priority
> > group have failed userspace multipathd will attempt to reload the
> > associated DM table to reflect that the device no longer has any
> > priority groups.  But the reload attempt always failed because the
> > multipath target did not allow 0 priority groups.
> > 
> > Adjust multipath target to allow a table with both 0 priority groups
> > and 0 for the initial priority group number.
> > 
> > All multipath target messages related to priority group (enable_group,
> > disable_group, switch_group) will properly handle a priority group of
> > 0 (will cause error).
> > 
> > When reloading a multipath table with 0 priority groups, userspace
> > multipathd must be updated to specify an initial priority group number
> > of 0 (rather than 1).
> 
>  Looks like we still have some action from multipath tool. CCing Ben..

Right, I looked in to the multipath-tools change.  Here is an RFC patch.

Inlined "FIXME"s pose the relevant questions.  I held off on having
select_path_group() return 0 when !mpp->pg because I wasn't confident
that I wouldn't break multipathd in some unintuitive way.

I did audit the various callers (indirect through setup_map call):

update_path_groups
ev_add_path
ev_remove_path

(all above first call setup_map)

setup_map()
  - mpp->bestpg = select_path_group()
  - assemble_map() -- establishes mp->params

do_map()

Even though I verified that much I wasn't completely confident in
changing select_path_group()'s defualt return of 1 -- so I localized the
change to where I _knew_ it was safe (assemble_map):

diff --git a/libmultipath/dmparser.c b/libmultipath/dmparser.c
index 1ef3aad..97b6420 100644
--- a/libmultipath/dmparser.c
+++ b/libmultipath/dmparser.c
@@ -52,6 +52,7 @@ assemble_map (struct multipath * mp)
 	int i, j;
 	int shift, freechar;
 	int minio;
+	int nr_priority_groups, initial_pg_nr;
 	char * p;
 	struct pathgroup * pgp;
 	struct path * pp;
@@ -60,9 +61,19 @@ assemble_map (struct multipath * mp)
 	p = mp->params;
 	freechar = sizeof(mp->params);
 
+	nr_priority_groups = VECTOR_SIZE(mp->pg);
+	/*
+	 * FIXME: make this conditional on multipath target version?
+	 * - but no existing code is conditional on multipath target version,
+	 *   nor does dm_drvprereq() store the version for code to do so.
+	 * - thing is older multipath never allowed nr_priority_groups=0 so
+	 *   it doesn't _really_ matter if initial_pg_nr=0 here...
+	 */
+	initial_pg_nr = (nr_priority_groups ? mp->bestpg : 0);
+
 	shift = snprintf(p, freechar, "%s %s %i %i",
 			 mp->features, mp->hwhandler,
-			 VECTOR_SIZE(mp->pg), mp->bestpg);
+			 nr_priority_groups, initial_pg_nr);
 
 	if (shift >= freechar) {
 		fprintf(stderr, "mp->params too small\n");
diff --git a/libmultipath/switchgroup.c b/libmultipath/switchgroup.c
index 025a95d..82c30ad 100644
--- a/libmultipath/switchgroup.c
+++ b/libmultipath/switchgroup.c
@@ -41,7 +41,7 @@ select_path_group (struct multipath * mpp)
 	struct pathgroup * pgp;
 
 	if (!mpp->pg)
-		return 1;
+		return 1; /* FIXME: return 0 here? */
 
 	vector_foreach_slot (mpp->pg, pgp, i) {
 		if (!pgp->paths)