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Date:   Fri, 22 Jul 2022 11:52:08 +0300
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Subject: Re: [PATCH]btrfs: Fix fstest case btrfs/219
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To:     hmsjwzb <hmsjwzb@zoho.com>, anand.jain@oracle.com
Cc:     Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20220721083609.5695-1-hmsjwzb@zoho.com>
 <0c2337c3-2b39-c54c-27e9-dd4f0a99cf71@suse.com>
 <1aaff0ac-436e-7782-081c-3549ff3d8c8f@zoho.com>
From:   Nikolay Borisov <nborisov@suse.com>
In-Reply-To: <1aaff0ac-436e-7782-081c-3549ff3d8c8f@zoho.com>
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On 22.07.22 г. 8:34 ч., hmsjwzb wrote:
> 
> 
> On 7/21/22 09:37, Nikolay Borisov wrote:
>>
>>
>> On 21.07.22 г. 11:36 ч., Flint.Wang wrote:
>>> Hi,
>>> fstest btrfs/291 failed.
>>>
>>> [How to reproduce]
>>> mkdir -p /mnt/test/219.mnt
>>> xfs_io -f -c "truncate 256m" /mnt/test/219.img1
>>> mkfs.btrfs /mnt/test/219.img1
>>> cp /mnt/test/219.img1 /mnt/test/219.img2
>>> mount -o loop /mnt/test/219.img1 /mnt/test/219.mnt
>>> umount /mnt/test/219.mnt
>>> losetup -f --show /mnt/test/219.img1 dev
>>> mount /dev/loop0 /mnt/test/219.mnt
>>> umount /mnt/test/219.mnt
>>> mount -o loop /mnt/test/219.img2 /mnt/test/219.mnt
>>>
>>> [Root cause]
>>> if (fs_devices->opened && found_transid < device->generation) {
>>>      /*
>>>       * That is if the FS is _not_ mounted and if you
>>>       * are here, that means there is more than one
>>>       * disk with same uuid and devid.We keep the one
>>>       * with larger generation number or the last-in if
>>>       * generation are equal.
>>>       */
>>>      mutex_unlock(&fs_devices->device_list_mutex);
>>>      return ERR_PTR(-EEXIST);
>>> }
>>>
>>> [Personal opinion]
>>> User might back up a block device to another. I think it is improper
>>> to forbid user from mounting it.
>>>
>>> Signed-off-by: Flint.Wang <hmsjwzb@zoho.com>
>>
>>
>> This lacks any explanation whatsoever so it's not possible to judge whether the fix is correct or not.
>>
>>> ---
>>>    fs/btrfs/volumes.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>>> index 6aa6bc769569a..76af32032ac85 100644
>>> --- a/fs/btrfs/volumes.c
>>> +++ b/fs/btrfs/volumes.c
>>> @@ -900,7 +900,7 @@ static noinline struct btrfs_device *device_list_add(const char *path,
>>>             * tracking a problem where systems fail mount by subvolume id
>>>             * when we reject replacement on a mounted FS.
>>>             */
>>> -        if (!fs_devices->opened && found_transid < device->generation) {
>>> +        if (fs_devices->opened && found_transid < device->generation) {
>>>                /*
>>>                 * That is if the FS is _not_ mounted and if you
>>>                 * are here, that means there is more than one
> 
> Hi Nikolay,
> 
> It seems the failure of btrfs/219 needs some explanation.
> 
> Here is the thing.
>          1. A storage device A with btrfs filesystem is running on a host.
>          2. For example, we backup the device A to an exactly some device B.
>          3. The device A continue to run for a while so the device->generation is getting bigger.
>          4. Then you umount the device A and try to mount device B.
>          5. Kernel find that device A has the same UUID as device B and has bigger device->generation.
>             So the mount request of device B will be rejected.
> 
>              if (!fs_devices->opened && found_transid < device->generation) {
>                   /*
>                    * That is if the FS is _not_ mounted and if you
>                    * are here, that means there is more than one
>                    * disk with same uuid and devid.We keep the one
>                    * with larger generation number or the last-in if
>                    * generation are equal.
>                    */
>                    mutex_unlock(&fs_devices->device_list_mutex);
>                    return ERR_PTR(-EEXIST);
>              }
> 
> I think it is improper to reject that request. Because device A is not in open state.

But then you will gravely confuse the filesystem about which device is 
the latest one, no? This code is rather old and the comments doesn't 
really help. So I'd like Chris (as the original author) to chime in on 
what the expected behavior should be ? IMO we shouldn't be allowing to 
add devices with older generation at all, irrespective of whether the fs 
is mounted or not.


> 
> Thanks