linux-kernel - Re: [PATCH 1/3] userfaultfd/selftests: fix feature support detection

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Date:   Tue, 21 Sep 2021 20:29:19 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Linux MM <linux-mm@...ck.org>,
        Linuxkselftest <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] userfaultfd/selftests: fix feature support detection

On Tue, Sep 21, 2021 at 01:31:12PM -0700, Axel Rasmussen wrote:
> Ah, it has to do with us asserting the list of expected ioctls. The
> kernel changes the list of ioctls it reports in response to a
> UFFDIO_REGISTER, depending on the particular kind of vma being
> registered, **as well as what mode(s) it is being registered with**.
> 
> So for example, consider the hugetlb_shared test. When registering,
> the kernel might set the UFFDIO_CONTINUE bit or not, depending on
> whether we registered with the MINOR mode bit set in particular.

I can understand your point, but the "capability set" of the kernel is still
the same.  In this case we should have UFFDIO_CONTINUE capability for
hugetlb_shared test globally, as long as the kernel supports it, irrelevant of
what test case we're going to have.

Then in the test, if we don't register with MINOR mode, IMHO we should just
mask out the expected_ioctls with UFFDIO_CONTINUE because it does not make
sense to request UFFDIO_CONTINUE if we will never use it in the test.

In other words, having a "uffd_features" global variable and having it changing
all the time during tests is odd to me, but I agree it's not a big deal. :)

-- 
Peter Xu