Re: [PATCH v1 0/4] setting uuid of online filesystems

[Catherine Hoang <catherine.hoang@oracle.com>]

> On Mar 13, 2023, at 11:28 PM, Dave Chinner <david@fromorbit.com> wrote:
> 
> On Mon, Mar 13, 2023 at 09:21:05PM -0700, Catherine Hoang wrote:
>> Hi all,
>> 
>> This series of patches implements a new ioctl to set the uuid of mounted
>> filesystems. Eventually this will be used by the 'xfs_io fsuuid' command
>> to allow userspace to update the uuid.
>> 
>> Comments and feedback appreciated!
> 
> What's the use case for this?

We want to be able to change the uuid on newly mounted clone vm images
so that each deployed system has a different uuid. We need to do this the
first time the system boots, but after the root fs is mounted so that fsuuid
can run in parallel with other service startup to minimize deployment times.
> 
> What are the pro's and cons of the implementation?
> 
> Some problems I see:
> 
> * How does this interact with pNFS exports (i.e.
> CONFIG_EXPORTFS_BLOCK_OPS). XFS hands the sb_uuid directly to pNFS
> server (and remote clients, I think) so that incoming mapping
> requests can be directed to the correct block device hosting the XFS
> filesystem the mapping information is for. IIRC, the pNFS data path
> is just given a byte offset into the device where the UUID in the
> superblock lives, and if it matches it allows the remote IO to go
> ahead - it doesn't actually know that there is a filesystem on that
> device at all...
> 
> * IIRC, the nfsd export table can also use the filesystem uuid to
> identify the filesystem being exported, and IIRC that gets encoded
> in the filehandle. Does changing the filesystem UUID then cause
> problems with export indentification and/or file handle
> creation/resolution?
> 
> * Is the VFS prepared for sb->s_uuid changing underneath running
> operations on mounted filesystems? I can see that this might cause
> problems with any sort of fscrypt implementation as it may encode
> the s_uuid into encryption keys held in xattrs, similarly IMA and
> EVM integrity protection keys.
> 
> * Should the VFS superblock sb->s_uuid use the XFS
> sb->sb_meta_uuid value and never change because we can encode it
> into other objects stored in the active filesystem? What does that
> mean for tools that identify block devices or filesystems by the VFS
> uuid rather than the filesystem uuid?
> 
> There's a whole can-o-worms here - the ability to dynamically change
> the UUID of a filesystem while it is mounted mean we need to think
> about these things - it's no longer as simple as "can only do it
> offline" which is typically only used to relabel a writeable
> snapshot of a golden image file during new VM deployment
> preparation.....
> 
>> 
>> Catherine
>> 
>> Catherine Hoang (4):
>>  xfs: refactor xfs_uuid_mount and xfs_uuid_unmount
>>  xfs: implement custom freeze/thaw functions
> 
> The "custom" parts that XFS requires need to be added to the VFS
> level freeze/thaw functions, not duplicate the VFS functionality
> within XFS and then slight modify it for this additional feature.
> Doing this results in unmaintainable code over the long term.
> 
>>  xfs: add XFS_IOC_SETFSUUID ioctl
>>  xfs: export meta uuid via xfs_fsop_geom
> 
> For what purpose does userspace ever need to know the sb_meta_uuid?

Userspace would need to know the meta uuid if we want to restore
the original uuid after it has been changed.
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com