[dm-devel] [PATCH] multipathd: avoid unnecessary path read-only reloads

[Benjamin Marzinski]

On Fri, Nov 19, 2021 at 09:33:39PM +0000, Martin Wilck wrote:
> On Thu, 2021-11-18 at 16:47 -0600, Benjamin Marzinski wrote:
> > A mulitpath device can only be reloaded read/write when all paths are
> > read/write. Also, whenever a read-only device is rescanned, the scsi
> > subsystem will first unconditionally issue a uevent with DISK_RO=0
> > before checking the read-only status, and if it the device is still
> > read-only, issuing another uevent with DISK_RO=1. These uevents cause
> > pointless rereads when read-only paths are rescanned. To avoid this,
> > check to see if all paths are read/write before changing a multipath
> > device from read-only to read/write.
> > 
> > Signed-off-by: Benjamin Marzinski <bmarzins at redhat.com>
> > ---
> >  libmultipath/libmultipath.version |  5 +++++
> >  libmultipath/sysfs.c              | 22 ++++++++++++++++++++++
> >  libmultipath/sysfs.h              |  1 +
> >  multipathd/main.c                 | 31
> > ++++++++++++++++++++++++++++++-
> >  4 files changed, 58 insertions(+), 1 deletion(-)
> > 
> > diff --git a/libmultipath/libmultipath.version
> > b/libmultipath/libmultipath.version
> > index 58a7d1be..ab4c7e30 100644
> > --- a/multipathd/main.c
> > +++ b/multipathd/main.c
> > @@ -1440,6 +1440,35 @@ finish_path_init(struct path *pp, struct
> > vectors * vecs)
> >         return -1;
> >  }
> >  
> > +static bool
> > +needs_ro_update(struct multipath *mpp, int ro)
> > +{
> > +       struct pathgroup * pgp;
> > +       struct path * pp;
> > +       unsigned int i, j;
> > +       struct dm_info *dmi = NULL;
> > +
> > +       if (!mpp || ro < 0)
> > +               return false;
> > +       dm_get_info(mpp->alias, &dmi);
> 
> Why can't you just use mpp->dmi here?

Since that value is set when the dmi is originally created, I didn't
want to not reload a map, if we simply haven't updated it yet to reflect
a change in the read-only value, like with do with dm_is_suspended()
or dm_get_deferred_remove(), etc. I could make a dm_get_read_only()
function and put it libmultipath/devmapper.c like the others, if you'd
rather.
 
> > +       if (!dmi) /* assume we do need to reload the device */
> > +               return true;
>
> Why that? I'd assume that if a DM_DEVICE_INFO ioctl fails on the
> device, a RELOAD would almost certainly fail, too.
> 

Since reloading when it's not necessary doesn't do any harm (it's what
we currently do) while not switching to read/write when we should is a
problem, I thought that I'd error on the side of caution, but I agree
that the reload is unlikey to succeed, so I can change this if you'd
like.

> > +       if (dmi->read_only == ro) {
> > +               free(dmi);
> > +               return false;
> > +       }
> > +       free(dmi);
> > +       if (ro == 1)
> > +               return true;
> > +       vector_foreach_slot (mpp->pg, pgp, i) {
> > +               vector_foreach_slot (pgp->paths, pp, j) {
> > +                       if (sysfs_get_ro(pp) == 1)
> > +                               return false;
> 
> I think you should also return false here if sysfs_get_ro() returns
> error.

Same thing here. I was erroring on the side of caution, but it should be
fine to change.

-Ben

 
> Regards,
> Martin
> 
> > +               }
> > +       }
> > +       return true;
> > +}
> > +
> >  static int
> >  uev_update_path (struct uevent *uev, struct vectors * vecs)
> >  {
> > @@ -1512,7 +1541,7 @@ uev_update_path (struct uevent *uev, struct
> > vectors * vecs)
> >                 }
> >  
> >                 ro = uevent_get_disk_ro(uev);
> > -               if (mpp && ro >= 0) {
> > +               if (needs_ro_update(mpp, ro)) {
> >                         condlog(2, "%s: update path write_protect to
> > '%d' (uevent)", uev->kernel, ro);
> >  
> >                         if (mpp->wait_for_udev)