linux-kernel - Re: [PATCH 1/6] KVM: guest_memfd: Add DEFAULT_SHARED flag, reject user page faults if not set

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Message-ID: <diqz4isl351g.fsf@google.com>
Date: Mon, 29 Sep 2025 10:51:39 +0000
From: Ackerley Tng <ackerleytng@...gle.com>
To: David Hildenbrand <david@...hat.com>, Patrick Roy <patrick.roy@...ux.dev>, 
	Fuad Tabba <tabba@...gle.com>, Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, "Kalyazin, Nikita" <kalyazin@...zon.co.uk>, shivankg@....com
Subject: Re: [PATCH 1/6] KVM: guest_memfd: Add DEFAULT_SHARED flag, reject
 user page faults if not set

David Hildenbrand <david@...hat.com> writes:

>                           GUEST_MEMFD_FLAG_DEFAULT_SHARED;
>>>>
>>>> At least for now, GUEST_MEMFD_FLAG_DEFAULT_SHARED and
>>>> GUEST_MEMFD_FLAG_MMAP don't make sense without each other. Is it worth
>>>> checking for that, at least until we have in-place conversion? Having
>>>> only GUEST_MEMFD_FLAG_DEFAULT_SHARED set, but GUEST_MEMFD_FLAG_MMAP,
>>>> isn't a useful combination.
>>>>
>>>
>>> I think it's okay to have the two flags be orthogonal from the start.
>> 
>> I think I dimly remember someone at one of the guest_memfd syncs
>> bringing up a usecase for having a VMA even if all memory is private,
>> not for faulting anything in, but to do madvise or something? Maybe it
>> was the NUMA stuff? (+Shivank)
>
> Yes, that should be it. But we're never faulting in these pages, we only 
> need the VMA (for the time being, until there is the in-place conversion).
>

Yup, Sean's patch disables faulting if GUEST_MEMFD_FLAG_DEFAULT_SHARED
is not set, but mmap() is always enabled so madvise() still works.

Requiring GUEST_MEMFD_FLAG_DEFAULT_SHARED to be set together with
GUEST_MEMFD_FLAG_MMAP would still allow madvise() to work since
GUEST_MEMFD_FLAG_DEFAULT_SHARED only gates faulting.

To clarify, I'm still for making GUEST_MEMFD_FLAG_DEFAULT_SHARED
orthogonal to GUEST_MEMFD_FLAG_MMAP with no additional checks on top of
whatever's in this patch. :)

> -- 
> Cheers
>
> David / dhildenb