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PH0PR10MB5706.namprd10.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Sep 2023 11:40:57.2867 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: p0E1E7t9/5fK1BODva5dlt5eSqHFXJrekfV6THkYi/BRYTBxfw3afi9vXb7tBFgysFN9cYrG3+2QFztJfoZVvw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA2PR10MB4588 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.267,Aquarius:18.0.980,Hydra:6.0.601,FMLib:17.11.176.26 definitions=2023-09-19_06,2023-09-19_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 mlxlogscore=999 adultscore=0 mlxscore=0 spamscore=0 malwarescore=0 suspectscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2308100000 definitions=main-2309190099 X-Proofpoint-GUID: G6Px1lNIrOEZ-wrC_SyoRWJb93TbAo4Z X-Proofpoint-ORIG-GUID: G6Px1lNIrOEZ-wrC_SyoRWJb93TbAo4Z Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 19/09/2023 06:44, 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. >> >> With this patch, now devices with the CHANGING_FSID_V2 flag are rejected. >> And its partner will fail to mount with the extra -o degraded option. >> >> Signed-off-by: Anand Jain >> --- >> Moreover, a btrfs-progs patch (below) has eliminated the use of the >> CHANGING_FSID_V2 flag entirely: >> >> [PATCH RFC] btrfs-progs: btrfstune -m|M remove 2-stage commit >> >> And we solve the compatability concerns as below: >> >> New-kernel new-progs - has no CHANGING_FSID_V2 flag. >> Old-kernel new-progs - has no CHANGING_FSID_V2 flag, kernel code unused. >> Old-kernel old-progs - bug may occur. >> New-kernel old-progs - Should use host with the newer btrfs-progs to fix. >> >> For legacy systems to help fix such a condition in the userspace instead >> we have the below patchset which ports of kernel's CHANGING_FSID_V2 code. >> >> [PATCH 00/16] btrfs-progs: recover from failed metadata_uuid >> >> And if it couldn't fix in some cases, users can use manually reunite, >> with the patchset: >> >> [PATCH 00/10] btrfs-progs: check and tune: add device and noscan options >> >> fs/btrfs/disk-io.c | 10 --- >> fs/btrfs/volumes.c | 166 ++++----------------------------------------- >> fs/btrfs/volumes.h | 1 - >> 3 files changed, 13 insertions(+), 164 deletions(-) > > Please split the kernel patch in two, one rejecting the CHANGING_FSID_V2 > bit and then removing the unused code. Yep. Will do. > I think the scanning code still > has to recognize the bit and skip the device, I haven't checked if > remains like that after this patch. Right. This patch does it. We only have to do it at device_list_add() because mount, device-scan, and device-ready all rely on device_list_add() to scan. Thanks, Anand