linux-kernel - Re: [PATCH v2 1/4] mm: Introduce vm_uffd

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Message-ID: <6a6c8b42-ed13-4fe2-9ef4-5641b81ed2d1@redhat.com>
Date: Mon, 22 Sep 2025 19:20:50 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Nikita Kalyazin <kalyazin@...zon.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Mike Rapoport
 <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Vlastimil Babka <vbabka@...e.cz>, Muchun Song <muchun.song@...ux.dev>,
 Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
 James Houghton <jthoughton@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Andrea Arcangeli <aarcange@...hat.com>, Oscar Salvador <osalvador@...e.de>,
 Axel Rasmussen <axelrasmussen@...gle.com>,
 Ujwal Kundur <ujwal.kundur@...il.com>
Subject: Re: [PATCH v2 1/4] mm: Introduce vm_uffd_ops API

On 18.09.25 20:20, Peter Xu wrote:
> On Thu, Sep 18, 2025 at 07:53:46PM +0200, David Hildenbrand wrote:
>> Re Nikita: If we could just reuse fault() for userfaultfd purposes, that
>> might actually be pretty nice.
> 
> I commented on that.
> 
> https://lore.kernel.org/all/aEiwHjl4tsUt98sh@x1.local/
> 
> That'll need to leak FAULT_FLAG_USERFAULT_CONTINUE which isn't necessary,
> make it extremely hard to know when to set the flag, and comlicates the
> fault path which isn't necessary.

I agree that FAULT_FLAG_USERFAULT_CONTINUE would be a very weird thing 
to have.

I was wondering whether it could be abstracted in a cleaner way, similar 
to what you described with the "NO_USERFAULT", but possibly taking it 
one step further (if possible ...).

In your reply you also mentioned "whether we can also avoid reusing 
fault() but instead resolve the page faults using the vm_ops hook too".

So it kind-of is a special type of page fault, but the question would be 
how that could be integrated more cleanly. And as you also point out, 
the question would be which other users it might really have.

In GUP we achieve not triggering userfaultfd by not setting 
FAULT_FLAG_ALLOW_RETRY.

But it's rather that not allowing to retry (drop+retake locks) makes it 
impossible to call into userfaultfd.

So not sure if abstracting/reusing that would make sense.

> 
> I think Mike's comment was spot on, that the new API is literally
> do_fault() for shmem, but only used in userfaultfd context so it's even an
> oneliner.

Right, special fault handling for userfaultfd.

> 
> I do not maintain mm, so above is only my two cents, so I don't make
> decisions.  Personally I still prefer the current approach of keep the mm
> main fault path clean.

Well, as clean as things like FAULT_FLAG_ALLOW_RETRY are. :)

In any case, reading Liams reply, it sounded like that there is a path 
forward, so I don't particularly care, just wanted to mention what I had 
in mind regarding FAULT_FLAG_ALLOW_RETRY.

-- 
Cheers

David / dhildenb