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

[Martin Wilck]

On Mon, 2021-11-22 at 09:35 -0600, Benjamin Marzinski wrote:
> 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.

I had expected that this property wouldn't silently change under us.
Actually, I do think that we should get an uevent if this happens.
Not sure if we process it properly, though.

>  
> > > +       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.

I misunderstood what being "cautious" means in this context. I gather
now that it's "if in doubt, reload". Fine with me.

> 
> > > +       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.

Same misunderstanding on my part. Thanks for the clarification.

Regards
Martin

> 
> -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)
>