Re: [PATCH v2] selftests: cover kernfs dentry revalidation

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Sep 04, 2026 at 10:13:11AM +0200, Christian Brauner wrote:
> On Tue, Sep 01, 2026 at 06:40:50PM -0700, Shakeel Butt wrote:
> > kernfs_test only checked two xattr calls. It did not touch the part of
> > kernfs that userspace hits most: the dentry cache.
> > 
> > The kernel adds and removes kernfs nodes on its own, with no VFS call
> > behind it, and removal cannot always unhash the dentries that are
> > already cached. So ->d_revalidate() is what keeps the cache honest,
> > let's add tests for it.
> > 
> > Add tests that make the kernel change kernfs directly, instead of going
> > through create and unlink:
> > 
> >  - writing cgroup.subtree_control makes the kernel add and remove files
> >    in every child cgroup. That checks two things: a cached negative
> >    dentry is dropped once the file appears, and a cached positive one
> >    is dropped once the file goes away.
> >  - renaming a network interface renames the sysfs node under a dentry
> >    that is already cached.
> >  - sysfs mounted in a new network namespace must list that namespace's
> >    interfaces and not the parent's, which covers the KERNFS_NS tagging
> >    code.
> >  - many lookups running against concurrent mkdir and rmdir. A lookup
> >    may only succeed or fail with an errno that means "it is gone".
> >  - open across removal, readdir returning each entry once, and a
> >    telldir()/seekdir() round trip, which uses the name hash cookie in
> >    kernfs_dir_pos().
> > 
> > Test the other direction too: walking dentries that are already cached
> > must not drop them. That is not just a slowdown. d_invalidate() calls
> > detach_mounts(), so a dentry dropped by mistake also unmounts whatever
> > is mounted under that directory.
> > 
> > Both filesystems are needed. cgroup2 has no ->rename and is not
> > namespace tagged. sysfs cannot be mounted with a set of children we
> > control. The config fragment stops a kernel built with kselftest-merge
> > from passing while it runs almost nothing.
> > 
> > To confirm the tests really catch a broken backstop, kernfs_dir_changed()
> > was stubbed to return false. 8 of the 11 tests then failed.
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> > ---
> 
> I have to say I would appreciate trimmed-down commit messages with less
> relative clauses. This reads very LLMy still. Especially for tests my
> appetite for wading through multiple paragraphs of info is very low.
> Same goes for the very wordy changelog below...

My appologies, I will be more precise and concise in future.

I assume the kernfs bug fixes [1] will also go to your vfs branch. I will send a
v2 of that series with appropriate tags and will try to improve the commit
messages as well. I have a followup improvements for kernfs_rwsem which I will
base on vfs branch. Just wanted to give a headsup.

Thanks a lot, Christian.

[1] https://lore.kernel.org/all/20260903040253.670020-1-shakeel.butt@linux.dev/