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Subject: Re: [RFC PATCH] kvm: Use huge pages for DAX-backed files
To:     Dan Williams <dan.j.williams@intel.com>,
        Barret Rhoden <brho@google.com>
Cc:     Dave Jiang <dave.jiang@intel.com>, zwisler@kernel.org,
        Vishal L Verma <vishal.l.verma@intel.com>,
        rkrcmar@redhat.com, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        KVM list <kvm@vger.kernel.org>,
        "Zhang, Yu C" <yu.c.zhang@intel.com>,
        "Zhang, Yi Z" <yi.z.zhang@intel.com>
References: <20181029210716.212159-1-brho@google.com>
 <CAPcyv4gJUjuSKwy7i2wuKR=Vz-AkDrxnGya5qkg7XTFxuXbtzw@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Wed, 31 Oct 2018 09:52:04 +0100
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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.

Paolo