Re: [ANNOUNCE] PUCK Agenda - 2024.08.07 - KVM userfault (guest_memfd/HugeTLB postcopy)

[Peter Xu]

On Mon, Aug 12, 2024 at 02:12:29PM +0000, Wang, Wei W wrote:
> In the example above, both UFFDIO_COPY and KVM_USERFAULT_COPY need to be
> invoked, e.g.:
> #1 invoke KVM_USERFAULT_COPY
> #2 invoke UFFDIO_COPY
> 
> This requires that UFFDIO_COPY does not conflict with KVM_USERFAULT_COPY. Current
> UFFDIO_COPY will fail (thus not waking up the threads on the waitq) when it fails to
> install the PTE into the page table (in the above example the PTE has already been
> installed into the page table by KVM_USERFAULT_COPY at #1).

Indeed, maybe we can fix that with an explicit UFFDIO_WAKE upon UFFDIO_COPY
failures iff -EEXIST (in this case, it should fall into "page cache exists"
category, even if pgtable can still be missing).

I assume OTOH a racy KVM_USERFAULT_COPY in whatever form doesn't need
anything but to kick the vcpu, irrelevant of whether the copy succeeded or
not.

Thanks,

-- 
Peter Xu