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

[Hannes Reinecke <hare@suse.de>]

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.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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