Re: [PATCH v2 7/7] kunit, slub: add test_kfree_rcu() and test_leak_destroy()

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Sun, Sep 22, 2024 at 11:13 PM Guenter Roeck <linux@roeck-us.net> wrote:
>
> On 9/21/24 23:16, Hyeonggon Yoo wrote:
> > On Sun, Sep 22, 2024 at 6:25 AM Vlastimil Babka <vbabka@suse.cz> wrote:
> >>
> >> On 9/21/24 23:08, Guenter Roeck wrote:
> >>> On 9/21/24 13:40, Vlastimil Babka wrote:
> >>>> +CC kunit folks
> >>>>
> >>>> On 9/20/24 15:35, Guenter Roeck wrote:
> >>>>> Hi,
> >>>>
> >>>> Hi,
> >>>>
> >>>>> On Wed, Aug 07, 2024 at 12:31:20PM +0200, Vlastimil Babka wrote:
> >>>>>> Add a test that will create cache, allocate one object, kfree_rcu() it
> >>>>>> and attempt to destroy it. As long as the usage of kvfree_rcu_barrier()
> >>>>>> in kmem_cache_destroy() works correctly, there should be no warnings in
> >>>>>> dmesg and the test should pass.
> >>>>>>
> >>>>>> Additionally add a test_leak_destroy() test that leaks an object on
> >>>>>> purpose and verifies that kmem_cache_destroy() catches it.
> >>>>>>
> >>>>>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> >>>>>
> >>>>> This test case, when run, triggers a warning traceback.
> >>>>>
> >>>>> kmem_cache_destroy TestSlub_kfree_rcu: Slab cache still has objects when called from test_leak_destroy+0x70/0x11c
> >>>>> WARNING: CPU: 0 PID: 715 at mm/slab_common.c:511 kmem_cache_destroy+0x1dc/0x1e4
> >>>>
> >>>> Yes that should be suppressed like the other slub_kunit tests do. I have
> >>>> assumed it's not that urgent because for example the KASAN kunit tests all
> >>>> produce tons of warnings and thus assumed it's in some way acceptable for
> >>>> kunit tests to do.
> >>>>
> >>>
> >>> I have all tests which generate warning backtraces disabled. Trying to identify
> >>> which warnings are noise and which warnings are on purpose doesn't scale,
> >>> so it is all or nothing for me. I tried earlier to introduce a patch series
> >>> which would enable selective backtrace suppression, but that died the death
> >>> of architecture maintainers not caring and people demanding it to be perfect
> >>> (meaning it only addressed WARNING: backtraces and not BUG: backtraces,
> >>> and apparently that wasn't good enough).
> >>
> >> Ah, didn't know, too bad.
> >>
> >>> If the backtrace is intentional (and I think you are saying that it is),
> >>> I'll simply disable the test. That may be a bit counter-productive, but
> >>> there is really no alternative for me.
> >>
> >> It's intentional in the sense that the test intentionally triggers a
> >> condition that normally produces a warning. Many if the slub kunit test do
> >> that, but are able to suppress printing the warning when it happens in the
> >> kunit context. I forgot to do that for the new test initially as the warning
> >> there happens from a different path that those that already have the kunit
> >> suppression, but we'll implement that suppression there too ASAP.
> >
> > We might also need to address the concern of the commit
> > 7302e91f39a ("mm/slab_common: use WARN() if cache still has objects on
> > destroy"),
> > the concern that some users prefer WARN() over pr_err() to catch
> > errors on testing systems
> > which relies on WARN() format, and to respect panic_on_warn.
> >
> > So we might need to call WARN() instead of pr_err() if there are errors in
> > slub error handling code in general, except when running kunit tests?
> >
>
> If people _want_ to see WARNING backtraces generated on purpose, so be it.
> For me it means that _real_ WARNING backtraces disappear in the noise.
> Manually maintaining a list of expected warning backtraces is too maintenance
> expensive for me, so I simply disable all kunit tests which generate
> backtraces on purpose. That is just me, though. Other testbeds may have
> more resources available and may be perfectly happy with the associated
> maintenance cost.
>
> In this specific case, I now have disabled slub kunit tests, and, as
> mentioned before, from my perspective there is no need to change the
> code just to accommodate my needs. I'll do the same with all other new
> unit tests which generate backtraces in the future, without bothering
> anyone.
>
> Sorry for the noise.

I don't think this was a noise :) IMO some people want to see WARNING
during testing to catch errors,
but not for the slub_kunit test case. I think a proper approach here
would be suppressing
warnings while running slub_kunit test cases, but print WARNING when
it is not running slub_kunit test cases.

That would require some work changing the slub error reporting logic
to print WARNING on certain errors.
Any opinions, Vlastimil?

Thanks,
Hyeonggon