Re: [PATCH v3 1/2] kunit: add a KUnit test for SLUB debugging functionality

[Marco Elver <elver@google.com>]

On Thu, 1 Apr 2021 at 21:04, Daniel Latypov <dlatypov@google.com> wrote:
...
> > > --- a/include/linux/slub_def.h
> > > +++ b/include/linux/slub_def.h
> > > @@ -133,6 +133,8 @@ struct kmem_cache {
> > >       unsigned int usersize;          /* Usercopy region size */
> > >
> > >       struct kmem_cache_node *node[MAX_NUMNODES];
> > > +
> > > +     int errors;                     /* Number of errors in cache */
> >
> > So, I think it's bad design to add a new field 'errors', just for the
> > test. This will increase kmem_cache size for all builds, which is
> > unnecessary.
> >
> > Is there use to retrieve 'errors' elsewhere?
> >
> > While you could guard this with #ifdef CONFIG_SLUB_DEBUG or so, there's
> > a better design option if this is just for the KUnit test's benefit: use
> > kunit_resource.
> >
> > The way it'd work is that for each test (you can add a common init
> > function) you add a named resource, in this case just an 'int' I guess,
> > that slab would be able to retrieve if this test is being run.
> >
> > In the test somewhere, you could add something like this:
> >
> >
> >         static struct kunit_resource resource;
> >         static int slab_errors;
> >
> >         ..........
> >
> >         static int test_init(struct kunit *test)
> >         {
> >                 slab_errors = 0;
> >                 kunit_add_named_resource(test, NULL, NULL, &resource,
> >                                          "slab_errors", &slab_errors);
> >                 return 0;
> >         }
> >
> >         ...... tests now check slab_errors .....
> >
> > and then in slub.c you'd have:
> >
> >         #if IS_ENABLED(CONFIG_KUNIT)
> >         static bool slab_add_kunit_errors(void)
> >         {
> >                 struct kunit_resource *resource;
> >
> >                 if (likely(!current->kunit_test))
> >                         return false;
> >                 resource = kunit_find_named_resource(current->kunit_test, "slab_errors");
> >                 if (!resource)
> >                         return false;
> >                 (*(int *)resource->data)++;
> >                 kunit_put_resource(resource);

      return true;

was missing.

> >         }
> >         #else
> >         static inline bool slab_add_kunit_errors(void) { return false; }
> >         #endif
> >
> > And anywhere you want to increase the error count, you'd call
> > slab_add_kunit_errors().
> >
> > Another benefit of this approach is that if KUnit is disabled, there is
> > zero overhead and no additional code generated (vs. the current
> > approach).
>
> The resource approach looks really good, but...
> You'd be picking up a dependency on
> https://lore.kernel.org/linux-kselftest/20210311152314.3814916-2-dlatypov@google.com/
> current->kunit_test will always be NULL unless CONFIG_KASAN=y &&
> CONFIG_KUNIT=y at the moment.
> My patch drops the CONFIG_KASAN requirement and opens it up to all tests.

Oh, that's a shame, but hopefully it'll be in -next soon.

> At the moment, it's just waiting another look over from Brendan or David.
> Any ETA on that, folks? :)
>
> So if you don't want to get blocked on that for now, I think it's fine to add:
>   #ifdef CONFIG_SLUB_KUNIT_TEST
>   int errors;
>   #endif

Until kunit fixes setting current->kunit_test, a cleaner workaround
that would allow to do the patch with kunit_resource, is to just have
an .init/.exit function that sets it ("current->kunit_test = test;").
And then perhaps add a note ("FIXME: ...") to remove it once the above
patch has landed.

At least that way we get the least intrusive change for mm/slub.c, and
the test is the only thing that needs a 2-line patch to clean up
later.

Thanks,
-- Marco