Re: [PATCH v2] diskfilter: use nodes in logical volume's segment as member device

[Daniel Kiper]

On Fri, Sep 17, 2021 at 03:34:32PM +0800, Michael Chang via Grub-devel wrote:
> On Wed, Sep 15, 2021 at 06:00:09PM +0200, Daniel Kiper wrote:
> > On Thu, Sep 09, 2021 at 09:02:29PM +0800, Michael Chang via Grub-devel 
> > wrote:
> > > Currently the grub_diskfilter_memberlist function returns all physical
> > > volumes added to a volume group to which a logical volume (LV) belongs.
> > > However this is suboptimal as it doesn't fit the intended behavior of
> > > returning underlying devices that make up the LV. To give a clear
> > > picture, the result should be identical to running commands below to
> > > display the logical volumes with underlying physical volumes in use.
> > >
> > >   localhost:~ # lvs -o lv_name,vg_name,devices /dev/system/root
> > >     LV   VG     Devices
> > >     root system /dev/vda2(512)
> > >
> > >   localhost:~ # lvdisplay --maps /dev/system/root
> > >     --- Logical volume ---
> > >       ...
> > >     --- Segments ---
> > >     Logical extents 0 to 4604:
> > >       Type                linear
> > >       Physical volume     /dev/vda2
> > >       Physical extents    512 to 5116
> > >
> > > As shown above, we can know system-root lv uses only /dev/vda2 to
> > > allocate it's extents, or we can say that /dev/vda2 is the member device
> > > comprising the system-root lv.
> > >
> > > It is important to be precise on the member devices, because that helps
> > > to avoid pulling in excessive dependency. Let's use an example to
> > > demonstrate why it is needed.
> > >
> > >   localhost:~ # findmnt /
> > >   TARGET SOURCE                  FSTYPE OPTIONS
> > >   /      /dev/mapper/system-root ext4   rw,relatime
> > >
> > >   localhost:~ # pvs
> > >     PV               VG     Fmt  Attr PSize    PFree
> > >     /dev/mapper/data system lvm2 a--  1020.00m    0
> > >     /dev/vda2        system lvm2 a--    19.99g    0
> > >
> > >   localhost:~ # cryptsetup status /dev/mapper/data
> > >   /dev/mapper/data is active and is in use.
> > >     type:    LUKS1
> > >     cipher:  aes-xts-plain64
> > >     keysize: 512 bits
> > >     key location: dm-crypt
> > >     device:  /dev/vdb
> > >     sector size:  512
> > >     offset:  4096 sectors
> > >     size:    2093056 sectors
> > >     mode:    read/write
> > >
> > >   localhost:~ # vgs
> > >     VG     #PV #LV #SN Attr   VSize  VFree
> > >     system   2   3   0 wz--n- 20.98g    0
> > >
> > >   localhost:~ # lvs -o lv_name,vg_name,devices
> > >     LV   VG     Devices
> > >     data system /dev/mapper/data(0)
> > >     root system /dev/vda2(512)
> > >     swap system /dev/vda2(0)
> > >
> > > We can learn from above that /dev/mapper/data is an encrypted volume and
> > > also gets assigned to volume group 'system' as one of it's physical
> > > volumes. And also it is not used by root device,
> > > /dev/mapper/system-root, for allocating extents, so it shouldn't be
> > > taking part in the process of setting up grub to access root device.
> > >
> > > However running grub-install reports error as volume group 'system'
> > > contains encrypted volume.
> > >
> > >   error: attempt to install to encrypted disk without cryptodisk
> > >   enabled. Set `GRUB_ENABLE_CRYPTODISK=y' in file `/etc/default/grub'.
> > >
> > > Certainly we can enable GRUB_ENABLE_CRYPTODISK=y and move on, but that
> > > is not always acceptable since the server may need to be booted
> > > unattended. Additionally, typing passphase for every system startup can
> > > be a big hassle of which most users would like to avoid.
> > >
> > > This patch solves the problem by returning exact physical volume,
> > > /dev/vda2, rightly used by system-root from the example above, thus
> > > grub-install will not error out because the excessive encrypted device
> > > to boot the root device is not configured.
> > >
> > > Signed-off-by: Michael Chang <mchang@suse.com>
> > > Tested-by: Olav Reinert <seroton10@gmail.com>
> > > ---
> > >  grub-core/disk/diskfilter.c | 60 ++++++++++++++++++++++++++-----------
> > >  1 file changed, 43 insertions(+), 17 deletions(-)
> > >
> > > diff --git a/grub-core/disk/diskfilter.c b/grub-core/disk/diskfilter.c
> > > index 6eb2349a6..3d02d56ec 100644
> > > --- a/grub-core/disk/diskfilter.c
> > > +++ b/grub-core/disk/diskfilter.c
> > > @@ -300,6 +300,8 @@ grub_diskfilter_memberlist (grub_disk_t disk)
> > >    grub_disk_dev_t p;
> > >    struct grub_diskfilter_vg *vg;
> > >    struct grub_diskfilter_lv *lv2 = NULL;
> > > +  struct grub_diskfilter_segment *seg;
> > > +  unsigned int i, j;
> > >
> > >    if (!lv->vg->pvs)
> > >      return NULL;
> > > @@ -331,27 +333,51 @@ grub_diskfilter_memberlist (grub_disk_t disk)
> > >       }
> > >      }
> > >
> > > -  for (pv = lv->vg->pvs; pv; pv = pv->next)
> > > -    {
> > > -      if (!pv->disk)
> > > +  for (i = 0, seg = lv->segments; i < lv->segment_count; i++, seg++)
> > > +    for (j = 0; j < seg->node_count; ++j)
> > > +      if (seg->nodes[j].pv != NULL)
> > >   {
> > > -   /* TRANSLATORS: This message kicks in during the detection of
> > > -      which modules needs to be included in core image. This happens
> > > -      in the case of degraded RAID and means that autodetection may
> > > -      fail to include some of modules. It's an installation time
> > > -      message, not runtime message.  */
> > > -   grub_util_warn (_("Couldn't find physical volume `%s'."
> > > -                     " Some modules may be missing from core image."),
> > > -                   pv->name);
> > > -   continue;
> > > +   pv = seg->nodes[j].pv;
> > > +
> > > +   if (!pv->disk)
> > > +     {
> > > +       /* TRANSLATORS: This message kicks in during the detection of
> > > +          which modules needs to be included in core image. This happens
> > > +          in the case of degraded RAID and means that autodetection may
> > > +          fail to include some of modules. It's an installation time
> > > +          message, not runtime message.  */
> >
> > Again, please fix formatting of this comment if you are moving it [1].
>
> Sorry, I should have double checked grub-dev document before sending the
> patch.
>
> I'll fix this in next version.

Cool! Thanks!

> > > +       grub_util_warn (_("Couldn't find physical volume `%s'."
> > > +                         " Some modules may be missing from core 
> > > image."),
> > > +                       pv->name);
> > > +       continue;
> > > +     }
> > > +
> > > +   for (tmp = list; tmp != NULL; tmp = tmp->next)
> > > +     if (!grub_strcmp (tmp->disk->name, pv->disk->name))
> > > +       break;
> > > +   if (tmp != NULL)
> > > +     continue;
> > > +
> > > +   tmp = grub_malloc (sizeof (*tmp));
> > > +   if (!tmp)
> > > +     goto fail;
> >
> > Please call grub_error() here.
>
> Hm. I think it is not necessary because grub_error() has been called by
> grub_malloc() when it fails, so we should just return NULL and don't try
> to override the error number and message already set.
>
> Even we can preserve grub_malloc's error via grub_error_push (), still
> it makes little sense to me to duplicate the effort here ...
>
> Would you please shed more light on this ?

OK, leave "goto fail;" as is...

Daniel