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

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Date:   Wed, 22 Sep 2021 13:32:56 -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

Hello, Axel,

On Wed, Sep 22, 2021 at 10:04:03AM -0700, Axel Rasmussen wrote:
> Thanks for discussing the design Peter. I have some ideas which might
> make for a nicer v2; I'll massage the code a bit and see what I can
> come up with.

Sure thing.  Note again that as I don't have a strong opinion on that, feel
free to keep it.  However if you provide v2, I'll read.

[off-topic below]

Another thing I probably have forgot but need your confirmation is, when you
worked on uffd minor mode, did you explicitly disable thp, or is it allowed?

When I'm reworking the uffd-wp series, I noticed that commit e1e267c7928f
("khugepaged: skip collapse if uffd-wp detected", 2020-04-07) was actually
awkward and not efficient, as we can simply lookup the vma flags for detecting
uffd-wp enablement.  I'm preparing a patch for it to do it by checking vmas
(and that patch will also pave the way for file-backed).

Then I noticed we need similar thing for minor mode?

I think the answer is yes, but I didn't see any code that explicitly handled
thp for minor mode, do you remember?

To be explicit, what if in mcontinue_atomic_pte() we get a shmem_getpage() call
with a thp returned?  Will minor mode break?

I plan to post the khugepaged patch soon and I plan to cover minor mode too
there, but I'm not sure whether that's enough, as the thp can be there from the
1st day I think, but I could have missed something.

-- 
Peter Xu