Merge patch series "fs: Fix missed inode write during fsync"

Jan Kara <jack@suse.cz> says:

The series strives to make sure sync_inode_metadata() ->
writeback_single_inode() will end up to properly persisting not only the inode
but also all metadata associated with the inode.  This is beneficial for
several reasons:

1) This removes the need for separate mmb_fsync() implementations. Filesystems
can just use simple_fsync().

2) This makes sure all metadata is written for IS_SYNC / IS_DIRSYNC inodes.
Currently most filesystems just write inode itself.

3) This fixes races when multiple fsyncs race for a while where mmb_sync()
could return before all buffers were really persisted (now I_SYNC state flag
properly serializes everything).

To be able to achieve this we add new .sync_inode_metadata superblock operation
which gets called from __writeback_single_inode() and new I_METADATA_WRITEBACK
state flag (putting everything into .write_inode is possible but it looked too
ugly so I've decided for the new operation).

This scheme with I_METADATA_WRITEBACK state flag also fixes the problem that
when WB_SYNC_NONE writeback happened between write(2) and fsync(2), fsync(2)
would fail to properly persist the inode. Note that this problem isn't specific
to filesystems using the generic metadata bh tracking. This patch set doesn't
fix such filesystems as it is already big enough but the universality of this
problem among simple filesystems is another reason why I've chosen the new
superblock operation instead of trying to somehow hack up metadata bh tracking
infrastructure.

* patches from https://patch.msgid.link/20260727101509.21667-1-jack@suse.cz:
  vfs: Remove mmb_fsync()
  fat: Replace fat_sync_inode() with sync_inode_metadata()
  fat: Fix missed inode writeback during fsync(2)
  ext4: Fix data integrity writeout issues in nojournal mode
  minix: Fix data integrity writeout issues
  bfs: Fix data integrity writeout issues
  udf: Fold udf_update_inode() into udf_write_inode()
  udf: Use sync_inode_metadata() in udf_evict_inode()
  udf: Drop udf_sync_inode()
  udf: Use sync_inode_metadata() to writeout IS_SYNC inode
  udf: Fix data integrity writeout issues
  ext2: Fix data integrity writeout issues
  ext2: Avoid unnecessary inode buffer writeback for sync(2)
  ext2: Drop __ext2_write_inode()
  ext2: Fix lost inode updates for IS_SYNC inodes
  fs: Provide way for filesystem to wait for metadata writeback
  ext4: Allocate mapping_metadata_bhs struct on demand
  fs: Fix missed inode writeback when racing with __writeback_single_inode
  fs: Fix possible UAF in mark_buffer_write_io_error()
  affs: Drop support for metadata bh tracking

Link: https://patch.msgid.link/20260727101509.21667-1-jack@suse.cz
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>