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

David Hildenbrand
2025-09-29T10:22:17Z

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