Re: [PATCH] selftests: cover kernfs dentry revalidation

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

On Fri, Aug 28, 2026 at 07:28:25PM -0700, Shakeel Butt wrote:
> kernfs_test only exercised two xattr calls, so nothing covered the part
> of kernfs most exposed to userspace: the dentry cache.
> 
> kernfs nodes appear and disappear from kernel contexts with no VFS
> operation driving them, and removal cannot reliably unhash the cached
> dentries, so ->d_revalidate() is the correctness backstop. It had no
> test at all.
> 
> Add tests that drive kernfs from kernel context rather than through VFS
> create/unlink:
> 
>  - writing cgroup.subtree_control makes the kernel add and remove files
>    in every child cgroup, exercising both the negative-dentry revision
>    check and the deactivation check,
>  - renaming a network interface renames the sysfs node underneath an
>    already cached dentry,
>  - sysfs in a fresh network namespace must show only that namespace's
>    interfaces, covering the KERNFS_NS tagging paths,
>  - lookup hammered against concurrent mkdir/rmdir must only ever see
>    success or an errno meaning "it went away",
>  - removal-while-open, readdir duplicate detection and a
>    seekdir()/telldir() round trip over kernfs_dir_pos()'s hash cookie.
> 
> Also the other direction: walking already cached dentries must not
> invalidate them. That is not merely a lost optimisation --
> d_invalidate() calls detach_mounts(), so a revalidation that wrongly
> fails silently tears down any mount underneath the directory.
> 
> Both filesystems are needed: cgroup2 has no ->rename and is not
> namespace tagged, and sysfs cannot be mounted with a controllable set of
> children. The config fragment keeps a kernel built via kselftest-merge
> from reporting a pass while running almost nothing.
> 
> Verified the suite reacts to a broken backstop rather than passing:
> stubbing kernfs_dir_changed() to return false turns 8 of the 11 tests
> red.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Sashiko raised couple of issues, so I will address those and send v2.