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

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

Sure.
Here is the link to the latest version of the patch, which is v4.
It is based on the mainline master.

https://patchwork.kernel.org/project/linux-btrfs/patch/65a11e853a31b18b620f31cbbddf03e277fe3edf.1709809171.git.anand.jain@oracle.com/

Thanks, Anand

On 3/8/24 20:02, Alex Romosan wrote:
> sorry about the previous html mail.
> 
> Just to eliminate any confusion, can you please provide either a link
> to v4 of the patch or include it in the reply to this and explicitly
> labeled as such? I am beginning to have doubts as to the version I was
> testing. Thanks
> 
> On Fri, Mar 8, 2024 at 2:52 PM Anand Jain <anand.jain@oracle.com> wrote:
>>
>>
>>
>> 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;
>>>>>         }
>>>>