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

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Message-ID: <aMxNgyVRuiFq2Sms@x1.local>
Date: Thu, 18 Sep 2025 14:20:51 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...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 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 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.

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.

Besides, this series also cleans up other places all over the places, the
vm_uffd_ops is a most simplified version of description for a memory type.
So IMHO it's beneficial in other aspects as well.  If uffd_copy() is a
concern, fine, we drop it.  We don't plan to have more use of UFFDIO_COPY
outside of the known three memory types after all.

Thanks,

-- 
Peter Xu