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

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Message-ID: <aGWMsfbayEco0j4R@x1.local>
Date: Wed, 2 Jul 2025 15:46:57 -0400
From: Peter Xu <peterx@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
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>,
	David Hildenbrand <david@...hat.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 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.

Having an abstraction layer for userfaultfd memtypes within mm always makes
some sense on its own to me, so as to remove separate random checks over
either shmem or hugetlb.  E.g. after the series applied, we can drop the
shmem header in userfaultfd code, which should also be a step forward.

Thanks,

>  
> > [1]
> > https://lore.kernel.org/all/114133f5-0282-463d-9d65-3143aa658806@amazon.com/
> > [2]
> > https://lore.kernel.org/all/7666ee96-6f09-4dc1-8cb2-002a2d2a29cf@amazon.com/
> > [3] https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git/commit/?h=kbuild&id=707f853d7fa3ce323a6875487890c213e34d81a0

-- 
Peter Xu