Re: [PATCH v4 RFC] btrfs: do not skip re-registration for the mounted device

[Anand Jain <anand.jain@oracle.com>]

On 3/8/24 17:45, Filipe Manana wrote:
> On Fri, Mar 8, 2024 at 2:28 AM Anand Jain <anand.jain@oracle.com> wrote:
>>
>> Filipe,
>>
>> We've received confirmation from the user that the original update-grub
>> issue has been fixed. Additionally, your reported issue using
>> './check btrfs/14[6-9] btrfs/15[8-9]' has been resolved.
>>
>> However, reproducing the bug has been inconsistent on my systems.
>> If you could try checking that as well, it would be appreciated.
> 
> Sure, but I'm lost as to what I should test.
> There are several patches, and multiple versions, in the mailing list.
> 
> What should be tested on top of the for-next branch?

v4 is the latest version of this patch, which is based on the mainline
master. As you reported that you were able to make btrfs/159 fail with
this patch at v2, v4 of this patch theoretically fixes the bug you
reported. So, I wanted to know if you are still able to reproduce
the bug with v4?

Test case:
./check btrfs/14[6-9] btrfs/15[8-9]

Thanks.

> 
> Thanks.
> 
>>
>> David,
>>
>> If everything is good with v4, would you like v5 with the RFC
>> removed and "CC: stable@vger.kernel.org # 6.7+" added? Or if
>> it could be done during integration? I'm fine either way.
>>
>> Thanks,
>> Anand
>>
>> On 3/7/24 16:38, 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.
>>>
>>> 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/
>>> Tested-by: Alex Romosan <aromosan@gmail.com>
>>> Tested-by: CHECK_1234543212345@protonmail.com
>>> Reviewed-by: David Sterba <dsterba@suse.com>
>>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>>> ---
>>> v4:
>>> I removed CC: stable@vger.kernel.org # 6.7+ as this is still in the RFC stage.
>>> I need this patch verified by the bug filer.
>>> Use devt from bdev->bd_dev
>>> Rebased on mainline kernel.org master branch
>>>
>>> v3:
>>> https://lore.kernel.org/linux-btrfs/e2add8d54fbbd813305ba014c11d21d297ad87d0.1709782041.git.anand.jain@oracle.com/T/#u
>>>
>>>    fs/btrfs/volumes.c | 57 ++++++++++++++++++++++++++++++++++++++--------
>>>    1 file changed, 47 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>>> index d67785be2c77..75bfef1b973b 100644
>>> --- a/fs/btrfs/volumes.c
>>> +++ b/fs/btrfs/volumes.c
>>> @@ -1301,6 +1301,47 @@ int btrfs_forget_devices(dev_t devt)
>>>        return ret;
>>>    }
>>>
>>> +static bool btrfs_skip_registration(struct btrfs_super_block *disk_super,
>>> +                                 const char *path, dev_t devt,
>>> +                                 bool mount_arg_dev)
>>> +{
>>> +     struct btrfs_fs_devices *fs_devices;
>>> +
>>> +     /*
>>> +      * Do not skip device registration for mounted devices with matching
>>> +      * maj:min but different paths. Booting without initrd relies on
>>> +      * /dev/root initially, later replaced with the actual root device.
>>> +      * A successful scan ensures update-grub selects the correct device.
>>> +      */
>>> +     list_for_each_entry(fs_devices, &fs_uuids, fs_list) {
>>> +             struct btrfs_device *device;
>>> +
>>> +             mutex_lock(&fs_devices->device_list_mutex);
>>> +
>>> +             if (!fs_devices->opened) {
>>> +                     mutex_unlock(&fs_devices->device_list_mutex);
>>> +                     continue;
>>> +             }
>>> +
>>> +             list_for_each_entry(device, &fs_devices->devices, dev_list) {
>>> +                     if ((device->devt == devt) &&
>>> +                         strcmp(device->name->str, path)) {
>>> +                             mutex_unlock(&fs_devices->device_list_mutex);
>>> +
>>> +                             /* Do not skip registration */
>>> +                             return false;
>>> +                     }
>>> +             }
>>> +             mutex_unlock(&fs_devices->device_list_mutex);
>>> +     }
>>> +
>>> +     if (!mount_arg_dev && btrfs_super_num_devices(disk_super) == 1 &&
>>> +         !(btrfs_super_flags(disk_super) & BTRFS_SUPER_FLAG_SEEDING))
>>> +             return true;
>>> +
>>> +     return false;
>>> +}
>>> +
>>>    /*
>>>     * Look for a btrfs signature on a device. This may be called out of the mount path
>>>     * and we are not allowed to call set_blocksize during the scan. The superblock
>>> @@ -1357,18 +1398,14 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags,
>>>                goto error_bdev_put;
>>>        }
>>>
>>> -     if (!mount_arg_dev && btrfs_super_num_devices(disk_super) == 1 &&
>>> -         !(btrfs_super_flags(disk_super) & BTRFS_SUPER_FLAG_SEEDING)) {
>>> -             dev_t devt;
>>> +     if (btrfs_skip_registration(disk_super, path, bdev_handle->bdev->bd_dev,
>>> +                                 mount_arg_dev)) {
>>> +             pr_debug("BTRFS: skip registering single non-seed device %s (%d:%d)\n",
>>> +                       path, MAJOR(bdev_handle->bdev->bd_dev),
>>> +                       MINOR(bdev_handle->bdev->bd_dev));
>>>
>>> -             ret = lookup_bdev(path, &devt);
>>> -             if (ret)
>>> -                     btrfs_warn(NULL, "lookup bdev failed for path %s: %d",
>>> -                                path, ret);
>>> -             else
>>> -                     btrfs_free_stale_devices(devt, NULL);
>>> +             btrfs_free_stale_devices(bdev_handle->bdev->bd_dev, NULL);
>>>
>>> -             pr_debug("BTRFS: skip registering single non-seed device %s\n", path);
>>>                device = NULL;
>>>                goto free_disk_super;
>>>        }
>>