Pull superblock callback update from Christian Brauner:
 "Currently all filesystems which implement super_operations::shutdown()
  can not afford losing a device.

  Thus fs_bdev_mark_dead() will just call the ->shutdown() callback for
  the involved filesystem.

  But it will no longer be the case, as multi-device filesystems like
  btrfs can handle certain device loss without the need to shutdown the
  whole filesystem.

  To allow those multi-device filesystems to be integrated to use
  fs_holder_ops:

   - Add a new super_operations::remove_bdev() callback

   - Try ->remove_bdev() callback first inside fs_bdev_mark_dead().

     If the callback returned 0, meaning the fs can handling the device
     loss, then exit without doing anything else.

     If there is no such callback or the callback returned non-zero
     value, continue to shutdown the filesystem as usual.

  This means the new remove_bdev() should only do the check on whether
  the operation can continue, and if so do the fs specific handlings.
  The shutdown handling should still be handled by the existing
  ->shutdown() callback.

  For all existing filesystems with shutdown callback, there is no
  change to the code nor behavior.

  Btrfs is going to implement both the ->remove_bdev() and ->shutdown()
  callbacks soon"

* tag 'vfs-6.17-rc1.super' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  fs: add a new remove_bdev() callback

Currently all filesystems which implement super_operations::shutdown()
can not afford losing a device.

Thus fs_bdev_mark_dead() will just call the ->shutdown() callback for the
involved filesystem.

But it will no longer be the case, as multi-device filesystems like
btrfs and bcachefs can handle certain device loss without the need to
shutdown the whole filesystem.

To allow those multi-device filesystems to be integrated to use
fs_holder_ops:

- Add a new super_operations::remove_bdev() callback

- Try ->remove_bdev() callback first inside fs_bdev_mark_dead()
  If the callback returned 0, meaning the fs can handling the device
  loss, then exit without doing anything else.

  If there is no such callback or the callback returned non-zero value,
  continue to shutdown the filesystem as usual.

This means the new remove_bdev() should only do the check on whether the
operation can continue, and if so do the fs specific handlings.
The shutdown handling should still be handled by the existing
->shutdown() callback.

For all existing filesystems with shutdown callback, there is no change
to the code nor behavior.

Btrfs is going to implement both the ->remove_bdev() and ->shutdown()
callbacks soon.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Link: https://lore.kernel.org/09909fcff7f2763cc037fec97ac2482bdc0a12cb.1752470276.git.wqu@suse.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

This function takes super_lock in shared mode, so it should release the
same lock.

Cc: stable@vger.kernel.org # v6.16-rc1
Fixes: af7551cf13cf7f ("super: remove pointless s_root checks")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Link: https://lore.kernel.org/20250611164044.GF6138@frogsfrogsfrogs
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Pull automount updates from Al Viro:
 "Automount wart removal

  A bunch of odd boilerplate gone from instances - the reason for
  those was the need to protect the yet-to-be-attched mount from
  mark_mounts_for_expiry() deciding to take it out.

  But that's easy to detect and take care of in mark_mounts_for_expiry()
  itself; no need to have every instance simulate mount being busy by
  grabbing an extra reference to it, with finish_automount() undoing
  that once it attaches that mount.

  Should've done it that way from the very beginning... This is a
  flagday change, thankfully there are very few instances.

  vfs_submount() is gone - its sole remaining user (trace_automount)
  had been switched to saner primitives"

* tag 'pull-automount' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  kill vfs_submount()
  saner calling conventions for ->d_automount()

Pull vfs freezing updates from Christian Brauner:
 "This contains various filesystem freezing related work for this cycle:

   - Allow the power subsystem to support filesystem freeze for suspend
     and hibernate.

     Now all the pieces are in place to actually allow the power
     subsystem to freeze/thaw filesystems during suspend/resume.
     Filesystems are only frozen and thawed if the power subsystem does
     actually own the freeze.

     If the filesystem is already frozen by the time we've frozen all
     userspace processes we don't care to freeze it again. That's
     userspace's job once the process resumes. We only actually freeze
     filesystems if we absolutely have to and we ignore other failures
     to freeze.

     We could bubble up errors and fail suspend/resume if the error
     isn't EBUSY (aka it's already frozen) but I don't think that this
     is worth it. Filesystem freezing during suspend/resume is
     best-effort. If the user has 500 ext4 filesystems mounted and 4
     fail to freeze for whatever reason then we simply skip them.

     What we have now is already a big improvement and let's see how we
     fare with it before making our lives even harder (and uglier) than
     we have to.

   - Allow efivars to support freeze and thaw

     Allow efivarfs to partake to resync variable state during system
     hibernation and suspend. Add freeze/thaw support.

     This is a pretty straightforward implementation. We simply add
     regular freeze/thaw support for both userspace and the kernel.
     efivars is the first pseudofilesystem that adds support for
     filesystem freezing and thawing.

     The simplicity comes from the fact that we simply always resync
     variable state after efivarfs has been frozen. It doesn't matter
     whether that's because of suspend, userspace initiated freeze or
     hibernation. Efivars is simple enough that it doesn't matter that
     we walk all dentries. There are no directories and there aren't
     insane amounts of entries and both freeze/thaw are already
     heavy-handed operations. If userspace initiated a freeze/thaw cycle
     they would need CAP_SYS_ADMIN in the initial user namespace (as
     that's where efivarfs is mounted) so it can't be triggered by
     random userspace. IOW, we really really don't care"

* tag 'vfs-6.16-rc1.super' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  f2fs: fix freezing filesystem during resize
  kernfs: add warning about implementing freeze/thaw
  efivarfs: support freeze/thaw
  power: freeze filesystems during suspend/resume
  libfs: export find_next_child()
  super: add filesystem freezing helpers for suspend and hibernate
  gfs2: pass through holder from the VFS for freeze/thaw
  super: use common iterator (Part 2)
  super: use a common iterator (Part 1)
  super: skip dying superblocks early
  super: simplify user_get_super()
  super: remove pointless s_root checks
  fs: allow all writers to be frozen
  locking/percpu-rwsem: add freezable alternative to down_read

Allow the power subsystem to support filesystem freeze for
suspend and hibernate.

For some kernel subsystems it is paramount that they are guaranteed that
they are the owner of the freeze to avoid any risk of deadlocks. This is
the case for the power subsystem. Enable it to recognize whether it did
actually freeze the filesystem.

If userspace has 10 filesystems and suspend/hibernate manges to freeze 5
and then fails on the 6th for whatever odd reason (current or future)
then power needs to undo the freeze of the first 5 filesystems. It can't
just walk the list again because while it's unlikely that a new
filesystem got added in the meantime it still cannot tell which
filesystems the power subsystem actually managed to get a freeze
reference count on that needs to be dropped during thaw.

There's various ways out of this ugliness. For example, record the
filesystems the power subsystem managed to freeze on a temporary list in
the callbacks and then walk that list backwards during thaw to undo the
freezing or make sure that the power subsystem just actually exclusively
freezes things it can freeze and marking such filesystems as being owned
by power for the duration of the suspend or resume cycle. I opted for
the latter as that seemed the clean thing to do even if it means more
code changes.

If hibernation races with filesystem freezing (e.g. DM reconfiguration),
then hibernation need not freeze a filesystem because it's already
frozen but userspace may thaw the filesystem before hibernation actually
happens.

If the race happens the other way around, DM reconfiguration may
unexpectedly fail with EBUSY.

So allow FREEZE_EXCL to nest with other holders. An exclusive freezer
cannot be undone by any of the other concurrent freezers.

Link: https://lore.kernel.org/r/20250329-work-freeze-v2-6-a47af37ecc3d@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

The last remaining user of vfs_submount() (tracefs) is easy to convert
to fs_context_for_submount(); do that and bury that thing, along with
SB_SUBMOUNT

Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Tested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

After commit 475d0db742e3 ("fs: Fix theoretical division by 0 in
super_cache_scan()."), there's no need to plus one to prevent
division by zero.

Remove it to simplify the code.

Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
Link: https://lore.kernel.org/20250428135050.267297-1-alexjlzheng@tencent.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Use a common iterator for all callbacks. We could go for something even
more elaborate (advance step-by-step similar to iov_iter) but I really
don't think this is warranted.

Link: https://lore.kernel.org/r/20250329-work-freeze-v2-5-a47af37ecc3d@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Use a common iterator for all callbacks.

Link: https://lore.kernel.org/r/20250329-work-freeze-v2-4-a47af37ecc3d@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>