[RFC PATCH v2 0/2] ext4: fast commit: fix lockdep issues

[Li Chen <me@linux.beauty>]

Hi,

This series fixes two lockdep issues in the ext4 fast commit paths.

1) ext4_fc_track_inode() can return without sleeping when
   EXT4_STATE_FC_COMMITTING is already clear. The lockdep assertion for
   i_data_sem should only fire when we actually go to sleep.

2) lockdep reports a possible deadlock due to lock order inversion
   between s_fc_lock and i_data_sem. The fast commit writer held s_fc_lock
   while writing the fast commit log. Writing the journal inode mapping
   can call ext4_map_blocks() and take i_data_sem, while metadata update
   paths can hold i_data_sem and call ext4_fc_track_inode() which takes
   s_fc_lock.

The fix drops s_fc_lock before the log writing step and uses
EXT4_STATE_FC_COMMITTING to keep inode and create dentry state stable
until cleanup.

Testing:
- QEMU VM, ext4 -O fast_commit on virtio-pmem + dax, verified both lockdep
  report reproduces on an older kernel and is gone with this series.

RFC v1 -> RFC v2:
 - patch 1: move comments to correct place
 - patch 2: add it to patchset.
 - add missing RFC prefix

RFC v1: https://lore.kernel.org/linux-ext4/20251222032655.87056-1-me@linux.beauty/T/#u

Li Chen (2):
  ext4: fast_commit: assert i_data_sem only before sleep
  ext4: fast commit: fix s_fc_lock vs i_data_sem inversion

 fs/ext4/fast_commit.c | 96 +++++++++++++++++++++++++++++++------------
 1 file changed, 69 insertions(+), 27 deletions(-)

-- 
2.51.0