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

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Message-ID: <e00c5f69-f4d7-859c-cbde-0392b0a5195c@redhat.com>
Date:   Wed, 31 Oct 2018 09:52:04 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Barret Rhoden <brho@...gle.com>
Cc:     Dave Jiang <dave.jiang@...el.com>, zwisler@...nel.org,
        Vishal L Verma <vishal.l.verma@...el.com>,
        rkrcmar@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        KVM list <kvm@...r.kernel.org>,
        "Zhang, Yu C" <yu.c.zhang@...el.com>,
        "Zhang, Yi Z" <yi.z.zhang@...el.com>
Subject: Re: [RFC PATCH] kvm: Use huge pages for DAX-backed files

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