On 10/21/2014 08:11 AM, Martin Schwidefsky wrote:
I agree with Dave (I thought I disagreed, but I changed my mind while
writing down my thoughts). Just define mm_forbids_zeropage in
arch/s390/include/asm, and make it return mm->context.use_skey---with a
comment explaining how this is only for processes that use KVM, and then
only for guests that use storage keys.
The mm_forbids_zeropage() sure will work for now, but I think a vma flag
is the better solution. This is analog to VM_MERGEABLE or VM_NOHUGEPAGE,
the best solution would be to only mark those vmas that are mapped to
the guest. That we have not found a way to do that yet in a sensible way
does not change the fact that "no-zero-page" is a per-vma property, no?
I agree it should be per-VMA. However, right now the code is
complicated unnecessarily by making it a per-VMA flag. Also, setting
the flag per VMA should probably be done in
kvm_arch_prepare_memory_region together with some kind of storage key
notifier. This is not very much like Dominik's patch. All in all,
mm_forbids_zeropage() provides a non-intrusive and non-controversial way
to fix the bug. Later on, switching to vma_forbids_zeropage() will be
trivial as far as mm/ code is concerned.
But if you insist we go with the mm_forbids_zeropage() until we find a
clever way to distinguish the guest vmas from the qemu ones.
Yeah, I think it is simpler for now.
Paolo
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