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

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Message-ID: <01886693-c73e-3696-860b-086417d695e1@intel.com>
Date:   Thu, 15 Dec 2016 07:34:33 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Li, Liang Z" <liang.z.li@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dgilbert@...hat.com" <dgilbert@...hat.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>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: Re: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast
 (de)inflating & fast live migration

On 12/14/2016 12:59 AM, Li, Liang Z wrote:
>> Subject: Re: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for
>> fast (de)inflating & fast live migration
>>
>> On 12/08/2016 08:45 PM, Li, Liang Z wrote:
>>> What's the conclusion of your discussion? It seems you want some
>>> statistic before deciding whether to  ripping the bitmap from the ABI,
>>> am I right?
>>
>> I think Andrea and David feel pretty strongly that we should remove the
>> bitmap, unless we have some data to support keeping it.  I don't feel as
>> strongly about it, but I think their critique of it is pretty valid.  I think the
>> consensus is that the bitmap needs to go.
>>
>> The only real question IMNHO is whether we should do a power-of-2 or a
>> length.  But, if we have 12 bits, then the argument for doing length is pretty
>> strong.  We don't need anywhere near 12 bits if doing power-of-2.
> 
> Just found the MAX_ORDER should be limited to 12 if use length instead of order,
> If the MAX_ORDER is configured to a value bigger than 12, it will make things more
> complex to handle this case. 
> 
> If use order, we need to break a large memory range whose length is not the power of 2 into several
> small ranges, it also make the code complex.

I can't imagine it makes the code that much more complex.  It adds a for
loop.  Right?

> It seems we leave too many bit  for the pfn, and the bits leave for length is not enough,
> How about keep 45 bits for the pfn and 19 bits for length, 45 bits for pfn can cover 57 bits
> physical address, that should be enough in the near feature. 
> 
> What's your opinion?

I still think 'order' makes a lot of sense.  But, as you say, 57 bits is
enough for x86 for a while.  Other architectures.... who knows?