fs: look up superblocks via the device table in fs_holder_ops

Switch the fs_holder_ops callbacks from recovering the single owning
superblock out of bdev->bd_holder to walking the device-to-superblock
table and acting on every superblock registered for the device. The
holder argument becomes purely the block layer's exclusivity token and
is no longer needed by the fs specific callbacks.

All devices opened with fs_holder_ops are registered by now: the main
device since setup_bdev_super() switched to fs_bdev_file_open_by_dev()
and the extra devices (xfs log and realtime devices, btrfs member
devices, the ext4 external journal) since the preceding per-filesystem
conversions. So no event is lost in the switchover.

The walk uses a refcount-pinning cursor: each step takes a reference on
the entry via sd_ref and resumes from its sd_node. Unlinking an entry
is deferred to the last unpin, so a cursor never resumes from a removed
node.

mark_dead and sync only need the passive reference the entry holds plus
s_umount, which they take with super_lock_shared(). freeze and thaw
additionally need an active reference and acquire it with
get_active_super(), which waits for the superblock to be born before
taking s_active. Taking s_active before the superblock is born would
pin a still-mounting superblock so a racing mount that aborts could
never drop s_active to zero and reach SB_DYING, deadlocking the wait
for SB_BORN. This is how filesystems_freeze() and filesystems_thaw()
acquire it too.

One semantic change: when no live superblock uses the device anymore
(the holder is dying or was never registered), fs_bdev_freeze() and
fs_bdev_thaw() now return 0 - freeze after syncing the block device -
where they used to return -EINVAL.

The freeze-deny release path moves to the table in the same switchover.
A device made unfreezable for a btrfs membership change must drop its
table entry before re-allowing freezing; otherwise a freeze racing the
release reaches the superblock through the still-registered entry and is
stranded once the release unlinks it. Split fs_bdev_unregister() out of
fs_bdev_file_release() - the inverse of fs_bdev_register() - so
btrfs_release_device_allow_freeze() can drop the {dev, sb} entry, re-allow
freezing on the still-open device, then close it. Re-allowing only after
the entry is gone keeps a racing freeze from reaching the superblock, and
doing it while the file is still open avoids touching the block device
after the close. btrfs previously yielded bd_holder before re-allowing,
which this commit makes irrelevant to freeze resolution.

Link: https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-12-7df6b864028e@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/fs/super.h b/include/linux/fs/super.h
index caf358483144..733d439f01ed 100644
--- a/include/linux/fs/super.h
+++ b/include/linux/fs/super.h
@@ -242,6 +242,7 @@ struct file *fs_bdev_file_open_by_dev(dev_t dev, blk_mode_t mode, void *holder,
 				      struct super_block *sb);
 struct file *fs_bdev_file_open_by_path(const char *path, blk_mode_t mode,
 				       void *holder, struct super_block *sb);
+void fs_bdev_unregister(struct file *bdev_file, struct super_block *sb);
 void fs_bdev_file_release(struct file *bdev_file, struct super_block *sb);
 
 #endif /* _LINUX_FS_SUPER_H */