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

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Message-ID: <aQPX859LbBg5FmE8@x1.local>
Date: Thu, 30 Oct 2025 17:26:11 -0400
From: Peter Xu <peterx@...hat.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Mike Rapoport <rppt@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Houghton <jthoughton@...gle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...e.com>,
	Ujwal Kundur <ujwal.kundur@...il.com>,
	Oscar Salvador <osalvador@...e.de>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v4 0/4] mm/userfaultfd: modulize memory types

Let me reply in short.

On Thu, Oct 30, 2025 at 04:24:46PM -0400, Liam R. Howlett wrote:
> Right, so the existing code a huge mess today and you won't fix
> anything, ever.

IMHO fix is the wrong word.  Cleanup it is.  I agree the current
userfaultfd code isn't extremely easy to follow.

> 
> We currently have a uffd_flags_t that sets wp or not then passes it
> through to set another part of the flag to pass it through again.  This
> is stupid.

I feel like you're applying very strongly personal preference to this piece
of code.  I won't say that is stupid.  Axel introduced it, and I think it's
still an improvement comparing to before.

> 
> And then it gets more stupid.  We then check that flag exactly once for
> the second argument in a global function and NEVER LOOK AT THAT PART
> AGAIN.  So we now have a type to contain a boolean, but we keep passing
> through the uffd_flags_t, so we can see if wp is set or not.
> 
> And you're fine with it.  In fact, let's add another one.  I mean, we
> tried for two but people didn't like two so lets whine and whine and
> whine until people get frustrated and let you push a second one of those
> gems into the code you REFUSED TO MAINTAIN.

What I refuse to maintain?  I don't really follow.

> 
> This is what owning a problem looks like: I removed the uufd_flags_t,
> because it's stupid.  I removed all the hugetlb checks because I
> modularized the memory types.
> 
> I'm happy to address changes, but I'm not happy to accept more
> middleware and "it's not part of the patch set" to address any problem
> as you push more trash into an already horrible code base.
> 
> We need to fix things too.
> 
> So I'm fixing it.

Let's wait for a 2nd opinion on the approaches.

As I said, I'm OK if everyone likes your solution and if I'm the only one
NACKing it.  If we can support guest-memfd finally whoever adds that, it's
not so bad.

-- 
Peter Xu