Re: [PATCH 4/6] btrfs-progs: use a unified btrfs_make_subvol() implementation

[David Sterba <dsterba@suse.cz>]

On Wed, Oct 18, 2023 at 10:20:57AM +1030, Qu Wenruo wrote:
> >>> We're moving towards a world where kernel-shared will be an exact-ish copy of
> >>> the kernel code.  Please put helpers like this in common/, I did this for
> >>> several of the extent tree related helpers we need for fsck, this is a good fit
> >>> for that.  Thanks,
> >>
> >> Sure, and this also reminds me to copy whatever we can from kernel.
> > 
> > I do syncs from kernel before a release but all the low hanging fruit is
> > probably gone so it needs targeted updates.
> 
> For the immediate target it's btrfs_inode and involved VFS structures 
> for inodes/dir entries.
> 
> In progs we don't have a structure to locate a unique inode (need both 
> rootid and ino number), nor to do any path resolution.
> 
> This makes it almost impossible to proper sync the code.
> 
> But introduce btrfs_inode to btrfs-progs would also be a little 
> overkilled, as we don't have that many users.
> (Only the new --rootdir with --subvol combination).

I have an idea for using this functionality, but you may not like it -
we could implement FUSE. The missing code is exactly about inodes, path
resolution and subvolumes. You have the other project, with a different
license, although there's a lot shared code. You can keep it so u-boot
can do the sync and keep the read-only support. I'd like to have full
read-write support with subvolumes and devices (if there's ioctl pass
through), but it's not urgent. Having the basic inode/path support would
be good for mkfs even in a smaller scope.