I'm still stuck trying to understand why subvolumes can't get their own superblocks: - Why are the performance issues Josef raises unsurmountable? And why are they unique to btrfs? (Surely there other cases where people need hundreds or thousands of superblocks?) - If filehandle decoding can return a different vfs mount than it's passed, why can't it return a different superblock? --b. On Wed, Jul 28, 2021 at 08:37:45AM +1000, NeilBrown wrote: > There are long-standing problems with btrfs subvols, particularly in > relation to whether and how they are exposed in the mount table. > > - /proc/self/mountinfo reports the major:minor device number for each > filesystem and when a btrfs subvol is explicitly mounted, the number > reported is wrong - it does not match what stat() reports for the > mountpoint. > > - when subvol are not explicitly mounted, they don't appear in > mountinfo at all. > > Consequences include that a tool which uses stat() to find the dev of the > filesystem, then searches mountinfo for that filesystem, will not find > it. > > Some tools (e.g. findmnt) appear to have been enhanced to cope with this > strangeness, but it would be best to make btrfs behave more normally. > > - nfsd cannot currently see the transition to subvol, so reports the > main volume and all subvols to the client as being in the same > filesystem. As inode numbers are not unique across all subvols, > this can confuse clients. In particular, 'find' is likely to report a > loop. > > subvols can be made to appear in mountinfo using automounts. However > nfsd does not cope well with automounts. It assumes all filesystems to > be exported are already mounted. So adding automounts to btrfs would > break nfsd. > > We can enhance nfsd to understand that some automounts can be managed. > "internal mounts" where a filesystem provides an automount point and > mounts its own directories, can be handled differently by nfsd. > > This series addresses all these issues. After a few enhancements to the > VFS to provide needed support, they enhance exportfs and nfsd to cope > with the concept of internal mounts, and then enhance btrfs to provide > them. > > The NFSv3 support is incomplete. I'm not sure we can make it work > "perfectly". A normal nfsv3 mount seem to work well enough, but if > mounted with '-o noac', it loses track of the mounted-on inode number > and complains about inode numbers changing. > > My basic test for these is to mount a btrfs filesystem which contains > subvols, nfs-export it and mount it with nfsv3 and nfsv4, then run > 'find' in each of the filesystem and check the contents of > /proc/self/mountinfo. > > The first patch simply fixes the dev number in mountinfo and could > possibly be tagged for -stable. > > NeilBrown > > --- > > NeilBrown (11): > VFS: show correct dev num in mountinfo > VFS: allow d_automount to create in-place bind-mount. > VFS: pass lookup_flags into follow_down() > VFS: export lookup_mnt() > VFS: new function: mount_is_internal() > nfsd: include a vfsmount in struct svc_fh > exportfs: Allow filehandle lookup to cross internal mount points. > nfsd: change get_parent_attributes() to nfsd_get_mounted_on() > nfsd: Allow filehandle lookup to cross internal mount points. > btrfs: introduce mapping function from location to inum > btrfs: use automount to bind-mount all subvol roots. > > > fs/btrfs/btrfs_inode.h | 12 +++ > fs/btrfs/inode.c | 111 ++++++++++++++++++++++++++- > fs/btrfs/super.c | 1 + > fs/exportfs/expfs.c | 100 ++++++++++++++++++++---- > fs/fhandle.c | 2 +- > fs/internal.h | 1 - > fs/namei.c | 6 +- > fs/namespace.c | 32 +++++++- > fs/nfsd/export.c | 4 +- > fs/nfsd/nfs3xdr.c | 40 +++++++--- > fs/nfsd/nfs4proc.c | 9 ++- > fs/nfsd/nfs4xdr.c | 106 ++++++++++++------------- > fs/nfsd/nfsfh.c | 44 +++++++---- > fs/nfsd/nfsfh.h | 3 +- > fs/nfsd/nfsproc.c | 5 +- > fs/nfsd/vfs.c | 162 +++++++++++++++++++++++---------------- > fs/nfsd/vfs.h | 12 +-- > fs/nfsd/xdr4.h | 2 +- > fs/overlayfs/namei.c | 5 +- > fs/xfs/xfs_ioctl.c | 12 ++- > include/linux/exportfs.h | 4 +- > include/linux/mount.h | 4 + > include/linux/namei.h | 2 +- > 23 files changed, 490 insertions(+), 189 deletions(-) > > -- > Signature
