[Libguestfs] [PATCH v2v] convert: Ignore /dev/mapper/osprober-* devices when trimming

[Richard W.M. Jones]

On Tue, May 17, 2022 at 07:54:01AM +0200, Laszlo Ersek wrote:
> On 05/16/22 17:46, Richard W.M. Jones wrote:
> > These devices can be left around by either grub2 or the osprober tool.
> > They are read-only mirrors of existing filesystems and it appears we
> > can safely ignore them.
> > 
> > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2003503
> > ---
> >  convert/convert.ml | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/convert/convert.ml b/convert/convert.ml
> > index 87fca7252b..997f6b08bd 100644
> > --- a/convert/convert.ml
> > +++ b/convert/convert.ml
> > @@ -194,10 +194,16 @@ and do_fstrim g inspect =
> >    (* Get all filesystems. *)
> >    let fses = g#list_filesystems () in
> >  
> > +  (* Ignore unknown/swap devices. *)
> >    let fses = List.filter_map (
> >      function (_, ("unknown"|"swap")) -> None | (dev, _) -> Some dev
> >    ) fses in
> >  
> > +  (* Ignore filesystems left around by osprober (RHBZ#2003503). *)
> > +  let fses =
> > +    List.filter (fun dev -> not (String.is_prefix dev "/dev/mapper/osprober-"))
> > +                fses in
> > +
> >    (* Trim the filesystems. *)
> >    List.iter (
> >      fun dev ->
> > 
> 
> I've read up on os-prober:
> 
> https://www.thegeekdiary.com/how-to-disable-os-prober-in-centos-rhel-7/
> https://github.com/campadrenalin/os-prober
> http://joeyh.name/code/os-prober/
> 
> and it's *absolutely infuriating* that os-prober itself, or whatever
> invokes os-prober, litters the system with block device nodes. In fact,
> I don't even know how it is *possible* for a device node to be left
> around under /dev/mapper -- I thought /dev/mapper/ would be re-populated
> *from zero* every time the system boots? Do we have something in the
> appliance perhaps that *creates* these files?

As part of conversion we run “/sbin/grub2-mkconfig -o /boot/grub2/grub.cfg”
which runs osprober and that leaves the block devices around.  Later
in conversion we list out all filesystems (ie. g#list_filesystems) and
try to trim these, which is what was giving the warnings.

This is only a workaround for the original bug which seems to be in
grub/osprober.

> I'm quite unhappy about ignoring these filesystems in "do_fstrim"
> *only*. If guestfs_list_filesystems() returns them in the first place,
> won't that cause confusion for other libguestfs applications or scripts?

Quite possibly, but I was fixing the smallest possible thing to make
the bug go away.

> Do we expect the same filesystem to appear multiply in the
> list-filesystems output?

During conversion, list_filesystems output is [tidied up]:

list_filesystems = [
  "/dev/mapper/osprober-linux-rhel_bootp--73--199--22-root", "xfs";
  "/dev/mapper/osprober-linux-sda1", "xfs";
  "/dev/sda1", "xfs";
  "/dev/sda3", "xfs";
  "/dev/rhel_bootp-73-199-22/root", "xfs";
  "/dev/rhel_bootp-73-199-22/swap", "swap";
  "/dev/rhel_bootp-73-199-6/root", "xfs";
  "/dev/rhel_bootp-73-199-6/swap", "swap"
]

(/dev/mapper/osprober-linux-sda1 & /dev/sda1), and
(/dev/mapper/osprober-linux-rhel_bootp--73--199--22-root & /dev/rhel_bootp-73-199-22/root)
are both duplicates.

It would be quite hard to deduplicate these however (unless you know
some trick), since the /dev/mapper/osprober-* devices are just
read-only device-mapper linear entries.  There's nothing apart from
the name that easily connects them to the partitions and LVs that they
cover (particularly the LV).

> I think this is a serious bug in some component different from
> libguestfs / virt-v2v.

Without a doubt, it's a bug in grub2/osprober.

> Hmmm... After reading your comment
> <https://bugzilla.redhat.com/show_bug.cgi?id=2003503#c12>, I think we
> absolutely need to report a bug for grub2. I'll contact the rhboot team.
> 
> I don't disagree that we have to work it around, but until we understand
> the problem better, I don't feel safe about filtering these nodes out
> only in virt-v2v's trimming function.

Sure, I'll leave this one for now.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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