linux-kernel - Re: [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

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Date:   Wed, 7 Dec 2016 08:57:01 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        "Li, Liang Z" <liang.z.li@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:     "mhocko@...e.com" <mhocko@...e.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "dgilbert@...hat.com" <dgilbert@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH kernel v5 0/5] Extend virtio-balloon for fast
 (de)inflating & fast live migration

Removing silly virtio-dev@ list because it's bouncing mail...

On 12/07/2016 08:21 AM, David Hildenbrand wrote:
>> Li's current patches do that.  Well, maybe not pfn/length, but they do
>> take a pfn and page-order, which fits perfectly with the kernel's
>> concept of high-order pages.
> 
> So we can send length in powers of two. Still, I don't see any benefit
> over a simple pfn/len schema. But I'll have a more detailed look at the
> implementation first, maybe that will enlighten me :)

It is more space-efficient.  We're fitting the order into 6 bits, which
would allows the full 2^64 address space to be represented in one entry,
and leaves room for the bitmap size to be encoded as well, if we decide
we need a bitmap in the future.

If that was purely a length, we'd be limited to 64*4k pages per entry,
which isn't even a full large page.