Re: [PATCH 2/2] kvm: Use huge pages for DAX-backed files

[Barret Rhoden <brho@google.com>]

On 2018-11-12 at 20:31 Paolo Bonzini <pbonzini@redhat.com> wrote:
> Looks good.  What's the plan for removing PageReserved from DAX pages?

I hear that's going on in this thread:

https://lore.kernel.org/lkml/154145268025.30046.11742652345962594283.stgit@ahduyck-desk1.jf.intel.com/

Though it looks like it's speeding up page initialization, and not
explicitly making the PageReserved change, yet.

Alternatively, I could change kvm_is_reserved_pfn() to single out
zone_device pages if we don't want to wait or if there is a concern
that it won't happen.

On a related note, there are two places in KVM where we check
PageReserved outside of kvm_is_reserved_pfn().

For reference:

bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
{
        if (pfn_valid(pfn))
                return PageReserved(pfn_to_page(pfn));

        return true;
}

One caller of PageReserved():

void kvm_set_pfn_dirty(kvm_pfn_t pfn)
{
        if (!kvm_is_reserved_pfn(pfn)) {
                struct page *page = pfn_to_page(pfn);

                if (!PageReserved(page))
                        SetPageDirty(page);
        }
}

In that one, the PageReserved() check looks redundant, since if the
page was PageReserved, then it would have been kvm_is_reserved.

The other is:

static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
{
        if (pfn_valid(pfn))
                return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
                        /*
                         * Some reserved pages, such as those from NVDIMM
                         * DAX devices, are not for MMIO, and can be mapped
                         * with cached memory type for better performance.
                         * However, the above check misconceives those pages
                         * as MMIO, and results in KVM mapping them with UC
                         * memory type, which would hurt the performance.
                         * Therefore, we check the host memory type in addition
                         * and only treat UC/UC-/WC pages as MMIO.
                         */
                        (!pat_enabled() || pat_pfn_immune_to_uc_mtrr(pfn));

        return true;
}

Where the PAT stuff was motivated by DAX.  The PageReserved check here
looks like a broken-out version of kvm_is_reserved_pfn(), so that we can
make some extra checks around it.

Anyway, I can get rid of those two PageReserved checks and/or have
kvm_is_reserved_pfn() just check DAX pages, if everyone is OK with that.

Thanks,

Barret