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Date:   Fri, 1 Jul 2022 12:04:41 +0200
From:   Vincent Whitchurch <vincent.whitchurch@axis.com>
To:     David Gow <davidgow@google.com>
CC:     Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Johannes Berg <johannes@sipsolutions.net>,
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        Jeff Dike <jdike@addtoit.com>,
        Richard Weinberger <richard@nod.at>,
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        Brendan Higgins <brendanhiggins@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
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Subject: Re: [PATCH v4 2/2] UML: add support for KASAN under x86_64
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On Fri, Jul 01, 2022 at 05:43:26PM +0800, David Gow wrote:
> On Fri, Jul 1, 2022 at 5:16 PM Vincent Whitchurch
> <vincent.whitchurch@axis.com> wrote:
> > On Fri, Jul 01, 2022 at 11:08:27AM +0200, David Gow wrote:
> > > On Thu, Jun 30, 2022 at 9:29 PM Andrey Konovalov <andreyknvl@gmail.com> wrote:
> > > > Stack trace collection code might trigger KASAN splats when walking
> > > > stack frames, but this can be resolved by using unchecked accesses.
> > > > The main reason to disable instrumentation here is for performance
> > > > reasons, see the upcoming patch for arm64 [1] for some details.
> > > >
> > > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?id=802b91118d11
> > >
> > > Ah -- that does it! Using READ_ONCE_NOCHECK() in dump_trace() gets rid
> > > of the nasty recursive KASAN failures we were getting in the tests.
> > >
> > > I'll send out v5 with those files instrumented again.
> >
> > Hmm, do we really want that?  In the patch Andrey linked to above he
> > removed the READ_ONCE_NOCHECK() and added the KASAN_SANITIZE on the
> > corresponding files for arm64, just like it's already the case in this
> > patch for UML.
> 
> Personally, I'm okay with the performance overhead so far: in my tests
> with a collection of ~350 KUnit tests, the total difference in runtime
> was about ~.2 seconds, and was within the margin of error caused by
> fluctuations in the compilation time.
> 
> As an example, without the stacktrace code instrumented:
> [17:36:50] Testing complete. Passed: 364, Failed: 0, Crashed: 0,
> Skipped: 47, Errors: 0
> [17:36:50] Elapsed time: 15.114s total, 0.003s configuring, 8.518s
> building, 6.433s running
> 
> versus with it instrumented:
> [17:35:40] Testing complete. Passed: 364, Failed: 0, Crashed: 0,
> Skipped: 47, Errors: 0
> [17:35:40] Elapsed time: 15.497s total, 0.003s configuring, 8.691s
> building, 6.640s running

OK, good to know.

> That being said, I'm okay with disabling it again and adding a comment
> if it's slow enough in some other usecase to cause problems (or even
> just be annoying). That could either be done in a v6 of this patchset,
> or a follow-up patch, depending on what people would prefer. But I'd
> not have a problem with leaving it instrumented for now.

I don't have any strong opinion either way either, so you don't have to
change it back on my account.  Thanks.