[PATCH RFC 0/8] fs: support freeze/thaw/mark_dead/sync with shared devices

[Christian Brauner <brauner@kernel.org>]

Note, this is on the border between RFC/POC and so I haven't pushed this
through testing yet. But I don't want to waste more time on this before
showing it.

I surveyed various fs implementations because I want the ability to
extend userspace the ability to manage what devices can be onlined in a
centralized way without having to force every fs to care about this.

I realized that erofs allows sharing block devices with multiple
superblocks. Any freeze, thaw, removal, or sync on those devices will
not be communicated to the superblocks using it and our current
infrastructure is unable to deal with this.

This attempts to add the ability to go from device number to all the
superblock using that device, iterate through them one-by-one and
perform actions on them. For most fses this is a 1:1 mapping but for
erofs its a 1:many mapping.

This is not unreasonable infastructure to support in my opinion. I
played around with some ideas for this and I want to send out an RFC to
gather some early input.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
Christian Brauner (8):
      fs, block: move blk_mode_t and fop_flags_t into <linux/types.h>
      fs: add a global device to super block hash table
      fs: refuse to claim any frozen block device
      xfs: port to fs_bdev_file_open_by_path()
      btrfs: open via dedicated fs bdev helpers
      ext4: open via dedicated fs bdev helpers
      erofs: open via dedicated fs bdev helpers
      super: make fs_holder_ops private

 fs/btrfs/dev-replace.c   |   6 +-
 fs/btrfs/ioctl.c         |   4 +-
 fs/btrfs/volumes.c       |  26 ++-
 fs/erofs/data.c          |   6 +
 fs/erofs/internal.h      |  10 ++
 fs/erofs/super.c         |  66 +++++--
 fs/erofs/zdata.c         |  10 +-
 fs/ext4/super.c          |  12 +-
 fs/super.c               | 452 ++++++++++++++++++++++++++++++++---------------
 fs/xfs/xfs_buf.c         |   2 +-
 fs/xfs/xfs_super.c       |  10 +-
 include/linux/blkdev.h   |   9 -
 include/linux/fs.h       |   2 -
 include/linux/fs/super.h |   7 +
 include/linux/types.h    |   2 +
 15 files changed, 433 insertions(+), 191 deletions(-)
---
base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
change-id: 20260602-work-super-bdev_holder_global-8cba5e52bed5