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

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Date:   Mon, 23 Oct 2023 18:36:16 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Suren Baghdasaryan <surenb@...gle.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 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]

-- 
Cheers,

David / dhildenb