Re: [PATCH v2] btrfs: do not skip re-registration for the mounted device

[David Sterba <dsterba@suse.cz>]

On Tue, Feb 13, 2024 at 09:13:56AM +0800, Anand Jain wrote:
> There are reports that since version 6.7 update-grub fails to find the
> device of the root on systems without initrd and on a single device.
> 
> This looks like the device name changed in the output of
> /proc/self/mountinfo:
> 
> 6.5-rc5 working
> 
>   18 1 0:16 / / rw,noatime - btrfs /dev/sda8 ...
> 
> 6.7 not working:
> 
>   17 1 0:15 / / rw,noatime - btrfs /dev/root ...
> 
> and "update-grub" shows this error:
> 
>   /usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?)
> 
> This looks like it's related to the device name, but grub-probe
> recognizes the "/dev/root" path and tries to find the underlying device.
> However there's a special case for some filesystems, for btrfs in
> particular.
> 
> The generic root device detection heuristic is not done and it all
> relies on reading the device infos by a btrfs specific ioctl. This ioctl
> returns the device name as it was saved at the time of device scan (in
> this case it's /dev/root).
> 
> The change in 6.7 for temp_fsid to allow several single device
> filesystem to exist with the same fsid (and transparently generate a new
> UUID at mount time) was to skip caching/registering such devices.
> 
> This also skipped mounted device. One step of scanning is to check if
> the device name hasn't changed, and if yes then update the cached value.
> 
> This broke the grub-probe as it always read the device /dev/root and
> couldn't find it in the system. A temporary workaround is to create a
> symlink but this does not survive reboot.
> 
> The right fix is to allow updating the device path of a mounted
> filesystem even if this is a single device one.
> 
> In the fix, check if the device's major:minor number matches with the
> cached device. If they do, then we can allow the scan to happen so that
> device_list_add() can take care of updating the device path. The file
> descriptor remains unchanged.
> 
> This does not affect the temp_fsid feature, the UUID of the mounted
> filesystem remains the same and the matching is based on device major:minor
> which is unique per mounted filesystem.
> 
> This covers the path when the device (that exists for all mounted
> devices) name changes, updating /dev/root to /dev/sdx. Any other single
> device with filesystem and is not mounted is still skipped.
> 
> Note that if a system is booted and initial mount is done on the
> /dev/root device, this will be the cached name of the device. Only after
> the command "btrfs device scan" it will change as it triggers the
> rename.
> 
> The fix was verified by users whose systems were affected.
> 
> CC: stable@vger.kernel.org # 6.7+
> Fixes: bc27d6f0aa0e ("btrfs: scan but don't register device on single device filesystem")
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=218353
> Link: https://lore.kernel.org/lkml/CAKLYgeJ1tUuqLcsquwuFqjDXPSJpEiokrWK2gisPKDZLs8Y2TQ@mail.gmail.com/
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> Tested-by: Alex Romosan <aromosan@gmail.com>
> Tested-by: CHECK_1234543212345@protonmail.com

Reviewed-by: David Sterba <dsterba@suse.com>

When you commit the patch, please reorder the tags according to
https://btrfs.readthedocs.io/en/latest/dev/Developer-s-FAQ.html#ordering