linux-kernel - Re: [RFC PATCH 0/5] KVM: guest_memfd: support for uffd missing

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Message-ID: <Z8i0HXen8gzVdgnh@x1.local>
Date: Wed, 5 Mar 2025 15:29:17 -0500
From: Peter Xu <peterx@...hat.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Nikita Kalyazin <kalyazin@...zon.com>, akpm@...ux-foundation.org,
	pbonzini@...hat.com, shuah@...nel.org, kvm@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, lorenzo.stoakes@...cle.com, david@...hat.com,
	ryan.roberts@....com, quic_eberman@...cinc.com, graf@...zon.de,
	jgowans@...zon.com, roypat@...zon.co.uk, derekmn@...zon.com,
	nsaenz@...zon.es, xmarcalx@...zon.com
Subject: Re: [RFC PATCH 0/5] KVM: guest_memfd: support for uffd missing

On Wed, Mar 05, 2025 at 11:35:27AM -0800, James Houghton wrote:
> I think it might be useful to implement an fs-generic MINOR mode. The
> fault handler is already easy enough to do generically (though it
> would become more difficult to determine if the "MINOR" fault is
> actually a MISSING fault, but at least for my userspace, the
> distinction isn't important. :)) So the question becomes: what should
> UFFDIO_CONTINUE look like?
> 
> And I think it would be nice if UFFDIO_CONTINUE just called
> vm_ops->fault() to get the page we want to map and then mapped it,
> instead of having shmem-specific and hugetlb-specific versions (though
> maybe we need to keep the hugetlb specialization...). That would avoid
> putting kvm/gmem/etc. symbols in mm/userfaultfd code.
> 
> I've actually wanted to do this for a while but haven't had a good
> reason to pursue it. I wonder if it can be done in a
> backwards-compatible fashion...

Yes I also thought about that. :)

When Axel added minor fault, it's not a major concern as it's the only fs
that will consume the feature anyway in the do_fault() path - hugetlbfs has
its own path to take care of.. even until now.

And there's some valid points too if someone would argue to put it there
especially on folio lock - do that in shmem.c can avoid taking folio lock
when generating minor fault message.  It might make some difference when
the faults are heavy and when folio lock is frequently taken elsewhere too.

It might boil down to how many more FSes would support minor fault, and
whether we would care about such difference at last to shmem users. If gmem
is the only one after existing ones, IIUC there's still option we implement
it in gmem code.  After all, I expect the change should be very under
control (<20 LOCs?)..

-- 
Peter Xu