Merge tag 'vfs-6.16-rc1.super' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

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

1 files changed, 15 insertions, 0 deletions

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index a82fbce25c28..c1719b5778a1 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -62,6 +62,21 @@ const struct super_operations kernfs_sops = {
 
 	.show_options	= kernfs_sop_show_options,
 	.show_path	= kernfs_sop_show_path,
+
+	/*
+	 * sysfs is built on top of kernfs and sysfs provides the power
+	 * management infrastructure to support suspend/hibernate by
+	 * writing to various files in /sys/power/. As filesystems may
+	 * be automatically frozen during suspend/hibernate implementing
+	 * freeze/thaw support for kernfs generically will cause
+	 * deadlocks as the suspending/hibernation initiating task will
+	 * hold a VFS lock that it will then wait upon to be released.
+	 * If freeze/thaw for kernfs is needed talk to the VFS.
+	 */
+	.freeze_fs	= NULL,
+	.unfreeze_fs	= NULL,
+	.freeze_super	= NULL,
+	.thaw_super	= NULL,
 };
 
 static int kernfs_encode_fh(struct inode *inode, __u32 *fh, int *max_len,