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: <aNq6Hz8U0BtjlgQn@google.com>
Date: Mon, 29 Sep 2025 09:55:59 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Ackerley Tng <ackerleytng@...gle.com>
Cc: David Hildenbrand <david@...hat.com>, Patrick Roy <patrick.roy@...ux.dev>, 
	Fuad Tabba <tabba@...gle.com>, 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, 
	Nikita Kalyazin <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

On Mon, Sep 29, 2025, Ackerley Tng wrote:
> 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.

Hah!  I totally intended that :-D

> 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. :)