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

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Message-ID: <289eede1-d47d-49a2-b9b6-ff8050d84893@redhat.com>
Date: Fri, 4 Jul 2025 11:34:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...nel.org>, Peter Xu <peterx@...hat.com>
Cc: Nikita Kalyazin <kalyazin@...zon.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 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>,
 "Liam R . Howlett" <Liam.Howlett@...cle.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 03.07.25 19:48, Mike Rapoport wrote:
> On Wed, Jul 02, 2025 at 03:46:57PM -0400, Peter Xu wrote:
>> On Wed, Jul 02, 2025 at 08:39:32PM +0300, Mike Rapoport wrote:
>>
>> [...]
>>
>>>> The main target of this change is the implementation of UFFD for
>>>> KVM/guest_memfd (examples: [1], [2]) to avoid bringing KVM-specific code
>>>> into the mm codebase.  We usually mean KVM by the "drivers" in this context,
>>>> and it is already somewhat "knowledgeable" of the mm.  I don't think there
>>>> are existing use cases for other drivers to implement this at the moment.
>>>>
>>>> Although I can't see new exports in this series, there is now a way to limit
>>>> exports to particular modules [3].  Would it help if we only do it for KVM
>>>> initially (if/when actually needed)?
>>>
>>> There were talks about pulling out guest_memfd core into mm, but I don't
>>> remember patches about it. If parts of guest_memfd were already in mm/ that
>>> would make easier to export uffd ops to it.
>>
>> Do we have a link to such discussion?  I'm also curious whether that idea
>> was acknowledged by KVM maintainers.
> 
> AFAIR it was discussed at one of David's guest_memfd calls

While it was discussed in the call a couple of times in different 
context (guest_memfd as a library / guest_memfd shim), I think we 
already discussed it back at LPC last year.

One of the main reasons for doing that is supporting guest_memfd in 
other hypervisors -- the gunyah hypervisor in the kernel wants to make 
use of it as well.

-- 
Cheers,

David / dhildenb