linux-kernel - Re: [PATCH v3 2/3] userfaultfd: UFFDIO

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Message-ID: <CAJuCfpF+ADAG=vn4y0EKo29j5zUx-TG=puF7Bq8oSp1_7T+OSw@mail.gmail.com>
Date:   Mon, 23 Oct 2023 10:33:40 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...hat.com,
        hughd@...gle.com, mhocko@...e.com, axelrasmussen@...gle.com,
        rppt@...nel.org, willy@...radead.org, Liam.Howlett@...cle.com,
        jannh@...gle.com, zhangpeng362@...wei.com, bgeffon@...gle.com,
        kaleshsingh@...gle.com, ngeoffray@...gle.com, jdduke@...gle.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI

On Mon, Oct 23, 2023 at 9:36 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 23.10.23 14:03, David Hildenbrand wrote:
> > On 22.10.23 17:46, Peter Xu wrote:
> >> On Fri, Oct 20, 2023 at 07:16:19PM +0200, David Hildenbrand wrote:
> >>> These are rather the vibes I'm getting from Peter. "Why rename it, could
> >>> confuse people because the original patches are old", "Why exclude it if it
> >>> has been included in the original patches". Not the kind of reasoning I can
> >>> relate to when it comes to upstreaming some patches.
> >>
> >> You can't blame anyone if you misunderstood and biased the question.
> >>
> >> The first question is definitely valid, even until now.  You guys still
> >> prefer to rename it, which I'm totally fine with.
> >>
> >> The 2nd question is wrong from your interpretation.  That's not my point,
> >> at least not starting from a few replies already.  What I was asking for is
> >> why such page movement between mm is dangerous.  I don't think I get solid
> >> answers even until now.
> >>
> >> Noticing "memcg is missing" is not an argument for "cross-mm is dangerous",
> >> it's a review comment.  Suren can address that.
> >>
> >> You'll propose a new feature that may tag an mm is not an argument either,
> >> if it's not merged yet.  We can also address that depending on what it is,
> >> also on which lands earlier.
> >>
> >> It'll be good to discuss these details even in a single-mm support.  Anyone
> >> would like to add that can already refer to discussion in this thread.
> >>
> >> I hope I'm clear.
> >>
> >
> > I said everything I had to say, go read what I wrote.
>
> Re-read your message after flying over first couple of paragraphs
> previously a bit quick too quickly (can easily happen when I'm told that
> I misunderstand questions and read them in a "biased" way).
>
> I'll happy to discuss cross-mm support once we actually need it. I just
> don't see the need to spend any energy on that right now, without any
> users on the horizon.
>
> [(a) I didn't blame anybody, I said that I don't understand the
> reasoning. (b) I hope I made it clear that this is added complexity (and
> not just currently dangerous) and so far I haven't heard a compelling
> argument why we should do any of that or even spend our time discussing
> that. (c) I never used "memcg is missing" as an argument for "cross-mm
> is dangerous", all about added complexity without actual users. (d) "it
> easily shows that there are cases  where this will require extra work --
> without any current benefits" -- is IMHO a perfectly fine argument
> against complexity that currently nobody needs]

Thanks for the discussion, folks!
I think posting the single-mm first and then following up with
cross-mm and its test would help us move forward. That will provide
functionality that is needed today quickly without unnecessary
distractions and will give us more time to discuss cross-mm support.
Also we will be able to test single-mm in isolation and make it more
solid before moving onto cross-mm.
I'll try to post the next version sometime this week.
Thanks,
Suren.

>
> --
> Cheers,
>
> David / dhildenb
>