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: <d2fa49af-112b-4de9-8c03-5f38618b1e57@redhat.com>
Date: Mon, 29 Sep 2025 12:22:12 +0200
From: David Hildenbrand <david@...hat.com>
To: Patrick Roy <patrick.roy@...ux.dev>, Ackerley Tng
 <ackerleytng@...gle.com>, 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

                          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).

-- 
Cheers

David / dhildenb