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

[Richard W.M. Jones]

On Tue, May 17, 2022 at 11:48:34AM +0200, Laszlo Ersek wrote:
> On 05/17/22 09:53, Richard W.M. Jones wrote:
> > 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 agree that we should not add complex logic here.
> 
> Do you think it's feasible to *move* the name-based filtering from
> virt-v2v's do_fstrim to the libguestfs daemon's list-filesystems API
> implementation? I can't see a reason why the list-filesystems API should
> *ever* return these /dev/mapper/osprober-* nodes.
> 
> IOW all I'm proposing is that we move the same workaround deeper in the
> stack.

Sure, that would work too, I'll come up with another patch soon.

Rich.

> Thanks!
> Laszlo
> 
> > 
> >> 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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