[PATCH v2 0/10] fs: Fix missed inode write during fsync

[Jan Kara <jack@suse.cz>]

Hello,

here is v2 of the patch series which fixes the possibly missing inode write
during fsync(2) for filesystems using generic metadata bh tracking. The
inherent problem is that .write_inode methods clear inode dirty bit but they
only copy inode contents into to the buffer cache. Because buffer carrying the
inode is shared among multiple inodes, it cannot be tracked by the generic
metadata bh tracking infrastructure and thus nothing is tracking that buffer
needs to be written out to maintain fsync(2) guarantees. Normally, this gets
taken care of by .write_inode checking for WB_SYNC_ALL writeback and submitting
& waiting for the buffer in that case however if flush worker ends up writing
the inode before data integrity writeback, this mechanism is broken.

This patch series adds a way for filesystems to track metadata block number
which contains the inode metadata and then uses this information to writeout
the buffer on fsync.

Changes since v1:
* Fixed freeing for ext4 dynamically allocated mmb struct
* Optimized tracking of block carrying the inode so that we don't flush it
  unnecessarily on fsync
* Add forgotten check for reclaimed bh to mmb_sync() to avoid NULL ptr deref
* Couple other smaller fixups pointed out by Sashiko

								Honza
Previous versions:
Link: http://lore.kernel.org/r/20260511115725.28441-1-jack@suse.cz # v1