[PATCH v6 0/17] fsfreeze: miscellaneous fixes and cleanups

["Fernando Luis Vázquez Cao" <fernando_b1@lab.ntt.co.jp>]

This patch set addresses several long standing issues:

- Fix emergency thaw (patch 1 through 4) and make some cleanups while at it
  (patches 5 and 6).

- Fix nested freezes for superblock-less block devices (patch 7).

- Integrate sb-level/bdev-level fsfreeze and sanitize the fsfreeze API to
  avoid perpetual frost in the event of changes in the filesystem namespace
  (patch 8 through 13).

- Add infrastructure needed to support dm-initiated freeze of filesystems
  spanning several bdevs and actually use it to implement this feature in
  btrfs (patch 14 through 16).

- Export freeze state to userspace through mountinfo (patch 17).

---
[1/17] vfs: add __iterate_supers() and helpers around it
[2/17] fsfreeze: add unlocked version of thaw_super

Preparatory patches to fix s_umount lockup of emergency thaw code.

[3/17] fsfreeze: fix emergency thaw infinite loop

Fix thaw_bdev so that it propagates the error code properly to the caller.
This bug caused emergency thaw to loop infinitely. This is a forward port of
a previous patch by Dave Chinner.

[4/17] fsfreeze: emergency thaw will deadlock on s_umount

Avoid emergency thaw deadlock on s_umount by using unlocked version of
thaw_super() and __iterate_supers() (introduced in patches 2 and 1
respectively).

[5/17] xfs: switch to using super methods for fsfreeze
[6/17] fsfreeze: move emergency thaw code to fs/super.c

Two cleanups (based on previous patches by David Chinner): convert xfs to using
superblock methods for fsfreeze and move emergency thaw code to where it
belongs.

[7/17] fsfreeze: fix nested freezing of sb-less bdevs

Get nested freezes working for sb-less bdevs.

[8/17] fsfreeze: allow bdev level thaws when the sb is unfrozen
[9/17] fsfreeze: freeze_super and thaw_bdev don't play well together

Restore the re-entrancy guards in sb-level API as the first step to achieve
proper sb-level/bdev-level fsfreeze integration.

[10/17] fsfreeze: automatically thaw on umount

The fsfreeze ioctls are a sb-level API which means that to thaw a frozen
filesystem it is necessary to have it in our filesystem hierarchy. To avoid
being stuck with a frozen filesystem and frozen processes by accident, we
automatically thaw filesystems on sys_umount().

[11/17] fsfreeze: add thaw_super_force

Both emergecy thaw and the umount-time automatic thaw need to make sure that
the thaw operation does not fail so with some code refactoring we can share
and clean up the code.

[12/17] fsfreeze: sb-level/bdev-level fsfreeze integration
[13/17] fsfreeze: unfreeze bdevs in addition to filesystems during emergency thaw

Fix the problem that a filesystem frozen through the bdev level API can be
thawed using the superblock level API and make sure that their respective
freeze counters are always consistent with each other.

[14/17] vfs: leverage bd_super in get_super and get_active_super
[15/17] btrfs: store pointer to superblock in bd_super
[16/17] fsfreeze: allow freeze counter lock nesting

Leverage bd_super in get_active_super to be able to get the superblock from the bdev when the filesystem spans several bdevs. Once this is in place, getting
thaw_bdev(), i.e. dm snapshot, and btrfs to play well together is just a matter
of using bd_super in btrfs.

[17/17] fsfreeze: export freeze_count through mountinfo

Export freeze_count through mountinfo so that we have a way to figure out the
current freeze state from userspace.
---

Changes since: v5:
  - Fix patch 4 ("fsfreeze: emergency thaw will deadlock on s_umount").
  - Add Jan Kara's Reviewed-by to patch 9 ("fsfreeze: freeze_super and
    thaw_bdev don't play well together").
  - Exclude conflictive check ioctls from the patch set (could be back in a
    follow-up patch set) and add a patch to export freeze state to userspace
    through mountinfo.
  - Add fix for nested freezing of sb-less bdevs.
  - Rewrite the patches to integrate sb-level/bdev-level fsfreeze and sanitize
    the fsfreeze API (including automatical thawing of filesystems on umount).
  - Add infrastructure needed to support dm-initiated freeze of filesystems
    spanning several bdevs and actually use it to implement this feature in
    btrfs.

Changes since v4:
  - Make implementation of iterate_supers_read and iterate_supers_write
    symmetric and update locking of emergency thaw code accordingly.
  - Add Jan Kara's Reviewed-by to patch 2.
  - Add explanation about the change from a block device based emergency thaw
    to a superblock based one to patch 4.
  - Modify patch 7 so that the new superblock based emergency does not leave
    the block device level fsfreeze counter in an inconsistent state.
  - Update and comment handling of fsfreeze during mount.

Changes since v3:
  - Include right version of the emergency thaw fix (thanks go to Josef for the
    heads up).
  - Add iterate_supers_(read/write) helpers as suggested by Eric Sandeen.
  - Fix typos.
  - Add Eric Sandeen's Reviewed-by to patches 2 and 3.

Changes since v2:
  - Rebase on top of Jan Kara's patch set which made it upstream.
  - Do without the horrible "emergency" parameter as suggested by Jan Kara.
  - New approach to fix emergency thaw deadlock on s_umount which does locking
    properly.

Changes since v1:
  - Do not break unfreezing of block devices without a mounted filesystem.
  - Avoid conditional locking in thaw_bdev.
  - Update filesystem locking documentation.