Re: [PATCH v1 2/3] overlay: Add the ability to remount volatile directories when safe

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Nov 25, 2020 at 10:31:36AM -0800, Sargun Dhillon wrote:
> On Wed, Nov 25, 2020 at 10:17 AM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Wed, Nov 25, 2020 at 02:46:20AM -0800, Sargun Dhillon wrote:
> >
> > [..]
> > > @@ -1125,16 +1183,19 @@ static int ovl_workdir_cleanup_recurse(struct path *path, int level)
> > >                       if (p->len == 2 && p->name[1] == '.')
> > >                               continue;
> > >               } else if (incompat) {
> > > -                     pr_err("overlay with incompat feature '%s' cannot be mounted\n",
> > > -                             p->name);
> > > -                     err = -EINVAL;
> > > -                     break;
> > > +                     err = ovl_check_incompat(ofs, p, path);
> > > +                     if (err < 0)
> > > +                             break;
> > > +                     /* Skip cleaning this */
> > > +                     if (err == 1)
> > > +                             continue;
> > >               }
> >
> > Shouldn't we clean volatile/dirty on non-volatile mount. I did a
> > volatile mount followed by a non-volatile remount and I still
> > see work/incompat/volatile/dirty and "trusted.overlay.volatile" xattr
> > on "volatile" dir. I would expect that this will be all cleaned up
> > as soon as that upper/work is used for non-volatile mount.
> >
> >
> 
> Amir pointed out this is incorrect behaviour earlier.
> You should be able to go:
> non-volatile -> volatile
> volatile -> volatile
> 
> But never
> volatile -> non-volatile, since our mechanism is not bulletproof.

Ok, so one needs to manually remove volatile/dirty to be able to
go from volatile to non-volatile.

I am wondering what does this change mean in terms of user visible
behavior. So far, if somebody tried a remount of volatile overlay, it 
will fail. After this change, it will most likely succeed. I am
hoping nobody relies on remount failure of volatile mount and
complain that user visible behavior changed after kernel upgrade.

Thanks
Vivek