Re: [RFC PATCH] kvm: Use huge pages for DAX-backed files

[Dan Williams]

On Wed, Oct 31, 2018 at 1:52 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 29/10/2018 23:25, Dan Williams wrote:
> > I'm wondering if we're adding an explicit is_zone_device_page() check
> > in this path to determine the page mapping size if that can be a
> > replacement for the kvm_is_reserved_pfn() check. In other words, the
> > goal of fixing up PageReserved() was to preclude the need for DAX-page
> > special casing in KVM, but if we already need add some special casing
> > for page size determination, might as well bypass the
> > kvm_is_reserved_pfn() dependency as well.
>
> No, please don't.  The kvm_is_reserved_pfn() check is for correctness,
> the page-size check is for optimization.  In theory you could have a
> ZONE_DEVICE area that is smaller than 2MB and thus does not use huge pages.

To be clear, I was not suggesting that a ZONE_DEVICE check would be
sufficient to determine a 2MB page. I was suggesting that given there
is a debate about removing the "reserved" designation for dax pages
that debate is moot if kvm is going to add interrogation code to
figure out the size of dax mappings.