fs: Provide way for filesystem to wait for metadata writeback

fs: Provide way for filesystem to wait for metadata writeback

Currently, inode and in general metadata writeback is handled in a lazy
manner. When inode is dirty, __writeback_single_inode() calls
.write_inode method which for lots of filesystems just copies inode
metadata into the underlying block buffer. Writeback of other metadata
associated with the inode (as well as buffers underlying inodes) is
usually handled completely separately and implicitely during writeback
of block device inode. This is good for efficiency of WB_SYNC_NONE
writeback or sync(2). However it becomes problematic for situations
where we want to make sure inode and its metadata is really persistent
on disk. fsync(2) is the most pronounced example of this and thus we
have grown a special file operation and various helper functions to
assist with this task. However fsync(2) is not the only case, For
example directories with DIRSYNC flag need similar functionality and
current use of sync_inode_metadata() for this task in filesystems
generally misses writeout of necessary metadata.

Furthermore even fsync(2) handling as implemented by simple_fsync() or
similar helpers is racy and can fail to properly persist the inode. The
problem is that WB_SYNC_NONE writeback can copy inode metadata into
underlying buffer and clean inode dirty bits. Following fsync(2) will
see inode is clean and will fail to make sure underlying buffer is
written out.

When multiple fsync(2) calls race, there's also another type of race
involving mmb_fsync(). There the problem is buffers already submitted to
the disk are no longer tracked in the mmb list and so racing mmb_sync()
can return before all of the IO completes.

Provide a new inode state bit I_METADATA_WRITEBACK tracking whether
writeback of inode related metadata may be needed for successful data
integrity sync and if this bit is set __writeback_single_inode() for
data integrity writeback will call new superblock operation
.sync_inode_metadata whose task is to make sure all metadata associated
with the inode (including the inode itself) is properly persisted. This
will allow filesystems to address the data integrity issues described
above and at the same time somewhat simplify our fsync implementations.
Issues with racing fsync(2) calls will be addressed by synchronization on
I_SYNC inode state which is set while calling .sync_inode_metadata, issues
with missed inode buffer writeback are fixed by filesystems looking up
corresponding buffer head and writing it out if needed.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260727104923.3828017-25-jack@suse.cz
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>