Re: [PATCH v4 0/4] mm/userfaultfd: modulize memory types

[David Hildenbrand (Red Hat)]

On 03.11.25 21:46, Peter Xu wrote:
> On Mon, Nov 03, 2025 at 09:01:02PM +0100, David Hildenbrand (Red Hat) wrote:
> > > > I have an extremely heavy workload at the moment anyway, but honestly
> > > > interactions like this have seriously put me off being involved in this review
> > > > personally.
> > > >
> > > > Do we really want this to be how review in mm or the kernel is?
> > > >
> > > > Is that really the culture we want to have here?
> > >
> > > Gosh.. Seriously?
> > >
> > > I'm ok if this needs to be audited.  I have all the previous discussions in
> > > the cover letter as links.
> >
> > I'm late to the party (or whatever this here is called. ah right, drama),
> > and I haven't yet dug through all the emails and certainly not through all
> > the of involved code changes.
> >
> > Peter, I was a bit surprised by your messages here indeed, not what I
> > expected.
> >
> > The "Your code allows to operate on pmd* in a module??? That's too risky and
> > mm can explode!  Isn't it?" definitely was absolutely unnecessary ... or
> > telling Liam that "he want almost mad".
>
> It was a joke!
>
> uffd_copy() API was NACKed because of this.  Now the new proposal
> introduced it.  I made a joke saying Liam allows that to happen in his
> branch, but forbid mine.
>
> I thought it was super clear to identify.

Text is a very bad medium for that, especially given the previous 
discussions that were rather heated.

So it's good that you clarify that -- I am not sure how many people got 
that it was a joke TBH.

I understood the reference to previous discussions but to me it sounded 
rather dismissive in the context of this discussion.

> > Again, not what I would have expected from you, and I would assume that you
> > had a bad day and would at least apologize now that some time passed.
>
> Sorry, no.  I won't apologize for that.  I was not fair treated, and now I
> think it's fair I at least make a joke.

Peter, if you would tell me that I am going mad I would not be able to 
understand that as a joke -- unless maybe if you add plenty of :) . :)

> David, you're leaving, and I'm totally dissappointed that at this point of
> time, you ask me to apologize instead.

I'll be right here, working for the community as I always do. So please 
read my message as if nothing in that regard happened.

I don't want you to feel bad here, I want us to find a solution without 
more of this drama.

Because that's what we have here, unfortunately :(

> I thought it was obvious a joke, because I never thought having pmd* in a
> function in a module is not OK.

Unfortunately it was not clear.

> I always thought it was fine, Linux is not a micro kernel.  It's just fine.
> It is what happening in Linux right now.  It is so obvious.  In case it was
> not clear, I hope I make it clear now.  If I'm going to formally NACK
> Liam's series, I won't use this as one of the real reasons.  I just hide it
> in some of others that are real reasons.  However if to be fair, when this
> reason is removed, this series should also remove the "highlight" that it
> removed shmem.h header, because my v1 also did that when with uffd_copy().
>
> > I understand that you were upset by the previous feedback on the earlier
> > series.
> >
> > There were some heated arguments in the last discussions, but most of them
> > were based on misunderstandings. I would have thought that once they were
> > resolved that we could continue focusing on discussing the technical details
> > again.
> >
> >  From what I can see you asked for actual code and when Liam came back with
> > some code that looks like *a lot of work* to me.
>
> It's Liam who stood out strongly pushing back what he at least used to be
> not familiar with.  This was, IMHO, rude.  It's ok to keep silent on some
> patchset that one isn't familiar.  It's ok to ask questions.  It's not ok
> to strongly push back without being extremely familiar with the code.

/me am I a rude person? :( ;)

The previous discussions on this were not ideal, because there were 
misunderstandings, yes. Liam has a lot of background on VMA handling, so 
I think getting is input is actually pretty valuable.

> He might be more familiar now, I wish he is. But it's Liam's decision to
> work on the code.

Right, Liam took the time to actually implement what he envisionsed. I 
assume it was a great learning experience for him.

Shame that this drama here seems to make him want to stop using that 
experience in the future.

> We're adults, we do what we should do, not what we asked to do.  If we do
> what we asked to do, we should have our reasons.
>
> My ask was trying to make Liam see that what he proposed is over
> engineering the whole thing.  I was pretty sure of that, he wasn't.  I
> explained to him multiple times on why it was an overkill, he doesn't
> agree. It's fine for him to disagree, it's Liam's right.  Then it's also
> fine for me to ask him code it up to notice himself, if I can't persuade
> him.  That's the only way for him to persuade me instead.

Well, he noticed that we can apparently cleanup userfaultfd quite 
heavily. :)

And maybe that's the main problem here: Liam talks about general uffd 
cleanups while you are focused on supporting guest_memfd minor mode "as 
simple as possible" (as you write below).

I acked your series for a reason: I think it is good enough to implement 
that (as simple as possible), but I also have the feeling that we can do 
much better in general.

> I sincerely wished that works out.  As I said, then I'll properly review
> it, and then we build whatever we need on top.  I'm totally fine.  However
> it didn't go like that, the API is exactly what I pictured.  I prefer my
> proposal.  That's what I did: showing the difference when there're two
> proposals, and ask for a second opinion.
>
> It's not fair to put that on top of me to blame.  He's trying to justify
> he's correct.  It has nothing to do with me.  He can stop pushing back
> anytime.  He can keep proposing what he works on.  It's his decision, not
> mine.

I would prefer if we can come to a conclusion instead of having people 
stop pushing back and walking away.

I assume positive intend here from both sides.

> > He really seems to care (which I highly appreciate) and went the extra mile
> > to show us how the uffd code could evolve.
> >
> > We've all (well okay, some of us) been crying for some proper userfaultfd
> > cleanups for years.
> >
> > So is there a way we can move forward with this without thinking in binary?
> > Is there some middle-ground to be had? Can some reworks come on top of your
> > series? Can so reworks be integrated in this series?
> >
> > I agree that what Liam proposes here is on the larger side, and probably
> > does a lot of things in a single rework. That doesn't mean that we couldn't
> > move into that direction in smaller steps.
> >
> > (I really don't think we should be thinking in terms of a CoC war like: show
> > them what I did and I will show them what they did. We are all working on
> > the same bigger goal here after all ...)
>
> We've got some second opinion from Mike, please read it first.

I read it, and I will have to look into some more details. But what I 
could read from Mikes reply is that there could be a discussion 
continuing where we would find a middle ground.

Well, if I can motivate Liam to keep working on userfaultfd at all.

 David,
> you're co-maintaining mm with Andrew.  I think it's fair indeed you provide
> how things should go together with Andrew.  It's fair you and Andrew
> whoever would like to make a decision on how to move forward.  I'm fine on
> whatever decision you want to make.

Unfortuantely (or fortunately?) I am not officially maintaining 
userfaultfd. And Andrew is not involved enough I am afraid to make a 
decision.

Of course, I *could* make a decision, but that would likely involve that 
we continue the discussion without this drama. But do people want that?

--
Cheers

David