linux-kernel - Re: [PATCH v3 4/5] guest_memfd: add support for userfaultfd minor mode

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Message-ID: <aS3f_PlxWLb-6NmR@x1.local>
Date: Mon, 1 Dec 2025 13:35:40 -0500
From: Peter Xu <peterx@...hat.com>
To: Nikita Kalyazin <kalyazin@...zon.com>
Cc: "David Hildenbrand (Red Hat)" <david@...nel.org>,
	Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Hugh Dickins <hughd@...gle.com>,
	James Houghton <jthoughton@...gle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Michal Hocko <mhocko@...e.com>, Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Shuah Khan <shuah@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 4/5] guest_memfd: add support for userfaultfd minor
 mode

On Mon, Dec 01, 2025 at 04:48:22PM +0000, Nikita Kalyazin wrote:
> I believe I found the precise point where we convinced ourselves that minor
> support was sufficient: [1].  If at this moment we don't find that reasoning
> valid anymore, then indeed implementing missing is the only option.
> 
> [1] https://lore.kernel.org/kvm/Z9GsIDVYWoV8d8-C@x1.local

Now after I re-read the discussion, I may have made a wrong statement
there, sorry.  I could have got slightly confused on when the write()
syscall can be involved.

I agree if you want to get an event when cache missed with the current uffd
definitions and when pre-population is forbidden, then MISSING trap is
required.  That is, with/without the need of UFFDIO_COPY being available.

Do I understand it right that UFFDIO_COPY is not allowed in your case, but
only write()?

One way that might work this around, is introducing a new UFFD_FEATURE bit
allowing the MINOR registration to trap all pgtable faults, which will
change the MINOR fault semantics.

That'll need some further thoughts, meanwhile we may also want to make sure
the old shmem/hugetlbfs semantics are kept (e.g. they should fail MINOR
registers if the new feature bit is enabled in the ctx somehow; or support
them properly in the codebase).

Thanks,

-- 
Peter Xu