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Thunderbird/102.9.1 Subject: Re: [PATCH] btrfs: reject device with CHANGING_FSID_V2 To: dsterba@suse.cz Cc: linux-btrfs@vger.kernel.org References: <02d59bdd7a8b778deb17e300354558498db59376.1692178060.git.anand.jain@oracle.com> <20230817120424.GK2420@twin.jikos.cz> Content-Language: en-US From: Anand Jain In-Reply-To: <20230817120424.GK2420@twin.jikos.cz> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SI2PR02CA0021.apcprd02.prod.outlook.com (2603:1096:4:195::10) To PH0PR10MB5706.namprd10.prod.outlook.com (2603:10b6:510:148::10) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH0PR10MB5706:EE_|BLAPR10MB4994:EE_ X-MS-Office365-Filtering-Correlation-Id: b50df4b4-0532-4bbb-eb9a-08db9f461c91 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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hVwZxd8QOYYSJnL25ab43FV0bKslossC X-Proofpoint-ORIG-GUID: hVwZxd8QOYYSJnL25ab43FV0bKslossC Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 17/8/23 20:04, David Sterba wrote: > On Wed, Aug 16, 2023 at 08:30:40PM +0800, Anand Jain wrote: >> The BTRFS_SUPER_FLAG_CHANGING_FSID_V2 flag indicates a transient state >> where the device in the userspace btrfstune -m|-M operation failed to >> complete changing the fsid. >> >> This flag makes the kernel to automatically determine the other >> partner devices to which a given device can be associated, based on the >> fsid, metadata_uuid and generation values. >> >> btrfstune -m|M feature is especially useful in virtual cloud setups, where >> compute instances (disk images) are quickly copied, fsid changed, and >> launched. Given numerous disk images with the same metadata_uuid but >> different fsid, there's no clear way a device can be correctly assembled >> with the proper partners when the CHANGING_FSID_V2 flag is set. So, the >> disk could be assembled incorrectly, as in the example below: >> >> Before this patch: >> >> Consider the following two filesystems: >> /dev/loop[2-3] are raw copies of /dev/loop[0-1] and the btrsftune -m >> operation fails. >> >> In this scenario, as the /dev/loop0's fsid change is interrupted, and the >> CHANGING_FSID_V2 flag is set as shown below. >> >> $ p="device|devid|^metadata_uuid|^fsid|^incom|^generation|^flags" >> >> $ btrfs inspect dump-super /dev/loop0 | egrep '$p' >> superblock: bytenr=65536, device=/dev/loop0 >> flags 0x1000000001 >> fsid 7d4b4b93-2b27-4432-b4e4-4be1fbccbd45 >> metadata_uuid bb040a9f-233a-4de2-ad84-49aa5a28059b >> generation 9 >> num_devices 2 >> incompat_flags 0x741 >> dev_item.devid 1 >> >> $ btrfs inspect dump-super /dev/loop1 | egrep '$p' >> superblock: bytenr=65536, device=/dev/loop1 >> flags 0x1 >> fsid 11d2af4d-1b71-45a9-83f6-f2100766939d >> metadata_uuid bb040a9f-233a-4de2-ad84-49aa5a28059b >> generation 10 >> num_devices 2 >> incompat_flags 0x741 >> dev_item.devid 2 >> >> $ btrfs inspect dump-super /dev/loop2 | egrep '$p' >> superblock: bytenr=65536, device=/dev/loop2 >> flags 0x1 >> fsid 7d4b4b93-2b27-4432-b4e4-4be1fbccbd45 >> metadata_uuid bb040a9f-233a-4de2-ad84-49aa5a28059b >> generation 8 >> num_devices 2 >> incompat_flags 0x741 >> dev_item.devid 1 >> >> $ btrfs inspect dump-super /dev/loop3 | egrep '$p' >> superblock: bytenr=65536, device=/dev/loop3 >> flags 0x1 >> fsid 7d4b4b93-2b27-4432-b4e4-4be1fbccbd45 >> metadata_uuid bb040a9f-233a-4de2-ad84-49aa5a28059b >> generation 8 >> num_devices 2 >> incompat_flags 0x741 >> dev_item.devid 2 >> >> >> It is normal that some devices aren't instantly discovered during >> system boot or iSCSI discovery. The controlled scan below demonstrates >> this. >> >> $ btrfs device scan --forget >> $ btrfs device scan /dev/loop0 >> Scanning for btrfs filesystems on '/dev/loop0' >> $ mount /dev/loop3 /btrfs >> $ btrfs filesystem show -m >> Label: none uuid: 7d4b4b93-2b27-4432-b4e4-4be1fbccbd45 >> Total devices 2 FS bytes used 144.00KiB >> devid 1 size 300.00MiB used 48.00MiB path /dev/loop0 >> devid 2 size 300.00MiB used 40.00MiB path /dev/loop3 >> >> /dev/loop0 and /dev/loop3 are incorrectly partnered. >> >> This kernel patch removes functions and code connected to the >> CHANGING_FSID_V2 flag. > > I didn't have a closer look but it seems you're removing all the logic > to make the metadata uuid robust and usable in case of interrupted > conversion, while finding another case where it does not work as you > expect. With this it would be change in behaviour, we need to check > the original use case. IIRC as the metadata uuid change is lightweight > we want to try harder to deal with the easy errors instead of rejecting > the filesystem mount. Robust indeed. Silently assembling wrong devices-data loss risk? Failing to assemble is still safe. I think it is better to introduce a sub-command to clone btrfs filesystem with a new device-uuid and same fsid (as it looks like same fsid has some use case). Thanks, Anand