From: Peter Xu <peterx@redhat.com>
To: Axel Rasmussen <axelrasmussen@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Shuah Khan <shuah@kernel.org>, Linux MM <linux-mm@kvack.org>,
Linuxkselftest <linux-kselftest@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/3] userfaultfd/selftests: fix feature support detection
Date: Tue, 21 Sep 2021 15:21:19 -0400 [thread overview]
Message-ID: <YUowr6phZU4v7dds@t490s> (raw)
In-Reply-To: <CAJHvVcj68inRrpmw0pJq9qFc20JzG8+s7b31HkXQcsLcAJN_0Q@mail.gmail.com>
On Tue, Sep 21, 2021 at 11:26:14AM -0700, Axel Rasmussen wrote:
> On Tue, Sep 21, 2021 at 10:44 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > Hi, Axel,
> >
> > On Tue, Sep 21, 2021 at 09:33:21AM -0700, Axel Rasmussen wrote:
> > > diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
> > > index 10ab56c2484a..2366caf90435 100644
> > > --- a/tools/testing/selftests/vm/userfaultfd.c
> > > +++ b/tools/testing/selftests/vm/userfaultfd.c
> > > @@ -79,10 +79,6 @@ static int test_type;
> > > #define ALARM_INTERVAL_SECS 10
> > > static volatile bool test_uffdio_copy_eexist = true;
> > > static volatile bool test_uffdio_zeropage_eexist = true;
> > > -/* Whether to test uffd write-protection */
> > > -static bool test_uffdio_wp = false;
> > > -/* Whether to test uffd minor faults */
> > > -static bool test_uffdio_minor = false;
> >
> > IMHO it's not a fault to have these variables; they're still the fastest way to
> > do branching. It's just that in some cases we should set them to "false"
> > rather than "true", am I right?
> >
> > How about we just set them properly in set_test_type? Say, we can fetch the
> > feature bits in set_test_type rather than assuming it's only related to the
> > type of memory.
>
> We could do that, but it would require opening a userfaultfd, issuing
> a UFFDIO_API ioctl, and getting the feature bits in set_test_type. And
> then I guess just closing the UFFD again, as we aren't yet setting up
> for any particular test. To me, it seemed "messier" than this
> approach.
>
> Another thing to consider is, for the next patch we don't just want to
> know "does this kernel support $FEATURE in general?" but also "is
> $FEATURE supported for this particular memory region I've
> registered?", and we can't have a single global answer to that.
Could I ask why? For each run, the memory type doesn't change, isn't it? Then
I think the capability it should support is a constant?
Btw, note that "open an uffd, detect features, close uffd quickly" during setup
phase is totally fine to me just for probing the capabilities, and instead of
thinking it being messy I see it a very clean approach..
> It seemed a bit cleaner to me to write the code as if I was dealing with that
> case, and then re-use the infrastructure I'd built for patch 2/3.
I didn't comment on patch 2, but I had the same confusion - aren't all these
information constant after we settle the hardware, the kernel and the memory
type to test?
>
> Basically, I didn't initially have a goal of getting rid of these
> variables, but it ended up being the cleanest way (IMHO).
>
> Just trying to explain the thinking. :) In the end, I think it's a
> stylistic choice and don't feel super strongly about it, either way
> could work. So, I can change it if you or others do feel strongly.
I have no strong opinion as long as the code works (which I trust you on :).
We can keep it in Andrew's queue unless you do feel the other way is better.
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2021-09-21 19:21 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-21 16:33 [PATCH 1/3] userfaultfd/selftests: fix feature support detection Axel Rasmussen
2021-09-21 16:33 ` [PATCH 2/3] userfaultfd/selftests: fix calculation of expected ioctls Axel Rasmussen
2021-09-21 16:33 ` [PATCH 3/3] userfaultfd/selftests: don't rely on GNU extensions for random numbers Axel Rasmussen
2021-09-21 18:03 ` Peter Xu
2021-09-21 17:44 ` [PATCH 1/3] userfaultfd/selftests: fix feature support detection Peter Xu
2021-09-21 18:26 ` Axel Rasmussen
2021-09-21 19:21 ` Peter Xu [this message]
2021-09-21 20:31 ` Axel Rasmussen
2021-09-22 0:29 ` Peter Xu
2021-09-22 17:04 ` Axel Rasmussen
2021-09-22 17:32 ` Peter Xu
2021-09-22 20:54 ` Axel Rasmussen
2021-09-22 21:51 ` Peter Xu
2021-09-22 22:29 ` Axel Rasmussen
2021-09-22 23:49 ` Peter Xu
2021-09-23 4:17 ` James Houghton
2021-09-23 5:43 ` Jue Wang
2021-09-24 20:09 ` Peter Xu
2021-09-24 20:22 ` Jue Wang
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