[PATCH v4 0/3] btrfs: ref_tracker for delayed_nodes

[Leo Martins <loemra.dev@gmail.com>]

The leading btrfs related crash in our fleet is a soft lockup in
btrfs_kill_all_delayed_nodes caused by a btrfs_delayed_node leak.
This patchset introduces ref_tracker infrastructure to detect this
leak. The feature is compiled in via CONFIG_BTRFS_DEBUG and enabled
via a ref_tracker mount option. I've run a full fstests suite with
ref_tracker enabled and experienced roughly a 7% slowdown in runtime.

Changelog:
v1: https://lore.kernel.org/linux-btrfs/fa19acf9dc38e93546183fc083c365cdb237e89b.1752098515.git.loemra.dev@gmail.com/
v2: https://lore.kernel.org/linux-btrfs/20250805194817.3509450-1-loemra.dev@gmail.com/T/#mba44556cfc1ae54c84255667a52a65f9520582b7
v3: https://lore.kernel.org/linux-btrfs/20250812113541.GD5507@twin.jikos.cz/T/#m264fd2f4ce786d9fbbb8a16b6762b8341f5ef902

v3->v4:
- remove CONFIG_BTRFS_FS_REF_TRACKER and use CONFIG_BTRFS_DEBUG

v2->v3:
- wrap ref_tracker and ref_tracker_dir in btrfs helper structs
- fix long function formatting
- new Kconfig CONFIG_BTRFS_FS_REF_TRACKER + ref_tracker mount option
- add a print to expose potential leaks
- move debug fields to the end of the struct

v1->v2:
- remove typedefs, now functions always take struct ref_tracker **
- put delayed_node::count back to original position to not change
  delayed_node struct size
- cleanup ref_tracker_dir if btrfs_get_or_create_delayed_node
  fails to create a delayed_node
- remove CONFIG_BTRFS_DELAYED_NODE_REF_TRACKER and use
  CONFIG_BTRFS_DEBUG

Leo Martins (3):
  btrfs: implement ref_tracker for delayed_nodes
  btrfs: print leaked references in kill_all_delayed_nodes
  btrfs: add mount option for ref_tracker

 fs/btrfs/Kconfig         |   3 +-
 fs/btrfs/delayed-inode.c | 193 ++++++++++++++++++++++++++++-----------
 fs/btrfs/delayed-inode.h |  93 +++++++++++++++++++
 fs/btrfs/fs.h            |   1 +
 fs/btrfs/super.c         |   7 ++
 5 files changed, 241 insertions(+), 56 deletions(-)

-- 
2.47.3