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.
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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
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2.53.0.414.gf7e9f6c205-goog
