[PATCH v4 0/3] kernfs: Add inotify IN_DELETE_SELF, IN_IGNORED support

["T.J. Mercier" <tjmercier@google.com>]

This series adds support for IN_DELETE_SELF and IN_IGNORED inotify
events to kernfs files and directories.

Currently, kernfs (used by cgroup and others) supports IN_MODIFY events
but fails to notify watchers when the file is removed (e.g. during
cgroup destruction). This forces userspace monitors to maintain resource
intensive side-channels like pidfds, procfs polling, or redundant
directory watches to detect when a cgroup dies and a watched file is
removed.

By generating IN_DELETE_SELF events on destruction, we allow watchers to
rely on a single watch descriptor for the entire lifecycle of the
monitored file, reducing resource usage (file descriptors, CPU cycles)
and complexity in userspace.

The series is structured as follows:
Patch 1 preemptively addresses a race to set/clear i_nlink that would
        arise in patch 2.
Patch 2 implements the logic to generate DELETE_SELF and IGNORED events
        on file / dir removal.
Patch 3 adds selftests to verify the new behavior.

---
Changes in v4:
Clear inode i_nlink upon kernfs removal instead of calling fsnotify
from kernfs per Jan. This adds support for directories.
Abandon support for files removed from vfs_writes.
Add selftest for directory watch per Amir.
Add Amir's Ack to selftests.

Changes in v3:
Remove parent IN_DELETE notification per Amir.
  Refactored kernfs_notify_workfn to avoid grabbing parent when
  unnecessary for DELETE events as a result.
Use notify_event for fsnotify_inode call per Amir
Initialize memcg pointers to NULL in selftests
Add Amir's Ack
Add Tejun's Acks to the series

Changes in v2:
Remove unused variables from new selftests per kernel test robot
Fix kernfs_type argument per Tejun
Inline checks for FS_MODIFY, FS_DELETE in kernfs_notify_workfn per Tejun

T.J. Mercier (3):
  kernfs: Don't set_nlink for directories being removed
  kernfs: Send IN_DELETE_SELF and IN_IGNORED
  selftests: memcg: Add tests for IN_DELETE_SELF and IN_IGNORED

 fs/kernfs/dir.c                               |  32 ++++-
 fs/kernfs/inode.c                             |   2 +-
 .../selftests/cgroup/test_memcontrol.c        | 112 ++++++++++++++++++
 3 files changed, 144 insertions(+), 2 deletions(-)


base-commit: ba268514ea14b44570030e8ed2aef92a38679e85
-- 
2.53.0.414.gf7e9f6c205-goog