Re: [PATCH v4 2/3] kernfs: Send IN_DELETE_SELF and IN_IGNORED

[Tejun Heo <tj@kernel.org>]

Hello, Amir.

On Fri, Feb 20, 2026 at 10:11:15PM +0200, Amir Goldstein wrote:
> > Yeah, that can be useful. For cgroupfs, there would probably need to be a
> > way to scope it so that it can be used on delegation boundaries too (which
> > we can require to coincide with cgroup NS boundaries).
> 
> I have no idea what the above means.
> I could ask Gemini or you and I prefer the latter ;)

Ah, you chose wrong. :)

> What are delegation boundaries and NFS boundaries in this context?

cgroup delegation is giving control of a subtree to someone else:

https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git/tree/Documentation/admin-guide/cgroup-v2.rst#n537

There's an old way of doing it by changing perms on some files and new way
using cgroup namespace.

> > Would it be possible to make FAN_MNT_ATTACH work for that?
> 
> FAN_MNT_ATTACH is an event generated on a mntns object.
> If "cgroup NS boundaries" is referring to a mntns object and if
> this object is available in the context of cgroup create/destroy
> then it should be possible.

Great, yes, cgroup namespace way should work then.

> But FAN_MNT_ATTACH reports a mountid. Is there a mountid
> to report on cgroup create? Probably not?

Sorry, I thought that was per-mount recursive file event monitoring.
FAN_MARK_MOUNT looks like the right thing if we want to allow monitoring
cgroup creations / destructions in a subtree without recursively watching
each cgroup.

Thanks.

-- 
tejun