Merge patch series "fs: support freeze/thaw/mark_dead/sync with shared devices"

Christian Brauner <brauner@kernel.org> says:

fs: support freeze/thaw/mark_dead/sync with shared devices

This is a generalization of the device number to superblock so it works
for actual block device and anonymous (or even mtd) devices.

fs_holder_ops recovers the affected superblock from bdev->bd_holder. That
forces the holder of a block device to be exactly one superblock and makes
it impossible for several superblocks to share a single device.

erofs does exactly that. It can mount read-only "blob" devices that are
shared between many superblocks: a metadata-only erofs that indexes a set
of per-layer blobs (one filesystem instead of one per OCI layer), or an
incremental image whose base device is shared by several updates. Because
the block layer only tracks a single holder, a freeze, thaw, removal or
sync on such a device is never propagated to all the superblocks using it,
and the current infrastructure has no way to find them.

This series replaces the bd_holder-based lookup with a global, dev_t-keyed
table mapping each block device to the superblock(s) using it. The holder
argument becomes purely the block layer's exclusivity token -- a superblock,
or the file_system_type for a device shared within one filesystem type --
and the fs_holder_ops callbacks look the device up in the table and act on
every superblock registered for it: 1:1 for most filesystems, 1:many for
erofs.

Filesystems claim and release their devices through new
fs_bdev_file_open_by_{dev,path}() and fs_bdev_file_release() helpers; the
per-fs patches convert xfs, btrfs, ext4, f2fs and erofs over to them and
fix cramfs and romfs, which released the registered main device with a
raw bdev_fput().

Since every superblock is registered under its s_dev the table also
replaces the last s_dev-keyed walk of the super_blocks list:
user_get_super() resolves device numbers through it, so ustat() and
quotactl() now work on any device a filesystem claims and no longer
take sb_lock.

The longer-term motivation is to let userspace decide which devices may be
onlined from one central place, without having to teach every filesystem
about it individually.

* patches from https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-0-7df6b864028e@kernel.org:
  selftests/filesystems: add ustat() coverage
  fs: look up the superblock via the device table in user_get_super()
  super: make fs_holder_ops private
  f2fs: open via dedicated fs bdev helpers
  erofs: open via dedicated fs bdev helpers
  fs: tolerate per-superblock freeze errors on shared devices
  fs: look up superblocks via the device table in fs_holder_ops
  ext4: open via dedicated fs bdev helpers
  btrfs: open via dedicated fs bdev helpers
  xfs: port to fs_bdev_file_open_by_path()
  fs: add dedicated block device open helpers for filesystems
  fs: maintain a global device-to-superblock table
  ocfs2: don't reset s_dev on dismount
  ext4: use anonymous devices for KUnit test superblocks
  fs, block: move blk_mode_t and fop_flags_t into <linux/types.h>
  super: take lock after last reference count
  super: convert s_count to refcount_t s_passive

Link: https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-0-7df6b864028e@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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