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Message-ID: <1aaff0ac-436e-7782-081c-3549ff3d8c8f@zoho.com>
Date:   Fri, 22 Jul 2022 01:34:11 -0400
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Subject: Re: [PATCH]btrfs: Fix fstest case btrfs/219
Content-Language: en-US
To:     Nikolay Borisov <nborisov@suse.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>
From:   hmsjwzb <hmsjwzb@zoho.com>
In-Reply-To: <0c2337c3-2b39-c54c-27e9-dd4f0a99cf71@suse.com>
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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.

Thanks