linux-kernel - Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

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Date:   Tue, 19 Feb 2019 21:21:20 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Rik van Riel <riel@...riel.com>, dodgen@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        dhildenb@...hat.com, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

On 19.02.19 21:17, Michael S. Tsirkin wrote:
> On Tue, Feb 19, 2019 at 09:02:52PM +0100, David Hildenbrand wrote:
>> On 19.02.19 20:58, Michael S. Tsirkin wrote:
>>> On Tue, Feb 19, 2019 at 10:06:35AM -0800, Alexander Duyck wrote:
>>>>> I tend to like an asynchronous reporting approach as discussed in this
>>>>> thread, we would have to see if Nitesh could get it implemented.
>>>>
>>>> I agree it would be great if it could work. However I have concerns
>>>> given that work on this patch set dates back to 2017, major issues
>>>> such as working around device assignment have yet to be addressed,
>>>
>>> BTW for device assignment to work, your idea of sending
>>> data directly to kvm won't work, will it?
>>> You need to update userspace so it can update VFIO right?
>>> Another blocker for assignment is ability to make holes
>>> an an existing mapping - supported by hardware but
>>> not by IOMMU drivers.
>>
>> I had the exact same thought and then realized that we decided to block
>> the balloon in user space until we figured out how to handle this properly.
>>
>> I wonder if MADV_FREE behaves differently compared to MADV_DONTNEED when
>> finding pinned pages, but I doubt it. Most probably we'll have to
>> disable hinting for device assignments as well.
> 
> OK but let's recognize it as a bug not a feature.
> 

Yes, btw interesting read: https://lwn.net/Articles/198380/

"Pages which have been locked into memory pose an extra challenge here -
they can be part of the page cache, but they still shouldn't be taken
away by the host system. So such pages cannot be marked as "volatile."
The problem is that figuring out if a page is locked is harder than it
might seem; it can involve scanning a list of virtual memory area (VMA)
structures, which is slow. So the hinting patches add a new flag to the
address_space structure to note that somebody has locked pages from that
address space in memory."

I assume locked here actually means pinned.

-- 

Thanks,

David / dhildenb