Re: [RFC PATCH] multipathd: use 0 for initial pg if nr_priority_groups=0

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Feb 01 2011 at 10:38am -0500,
Hannes Reinecke <hare@suse.de> wrote:

> On 01/31/2011 07:34 PM, Mike Snitzer wrote:
> > 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)
> > 
> I have a similar patchset in my multipath-tools tree, to allow maps
> with zero paths, too.
> However, there are quite a few places which required checking, as
> we're basically _never_ check if a pointer is NULL before accessing
> it. With a bit of luck I'll find some time to send out the patchset.
> 
> Otherwise you could have a look at
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/hare/multipath-tools.git
> branch sles11-sp1 will have the most recent stuff.

I found the following commit but it doesn't have any meaningful changes:
1e53326 Allow zero paths for device-mapper strings

Could you be more explicit on which commits you're thinking of?