linux-kernel - Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

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Message-ID: <5340d876-62b8-8a64-aa6d-7736c2c8710f@collabora.com>
Date:   Mon, 15 Aug 2022 14:50:20 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Christian König <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>, Huang Rui <ray.huang@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        Trigger Huang <Trigger.Huang@...il.com>,
        Gert Wollny <gert.wollny@...labora.com>,
        Antonio Caggiano <antonio.caggiano@...labora.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>, kvm@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

On 8/15/22 14:28, Christian König wrote:
>>>> Maybe it was discussed privately? In this case I will be happy to get
>>>> more info from you about the root of the problem so I could start to
>>>> look at how to fix it properly. It's not apparent where the problem is
>>>> to a TTM newbie like me.
>>>>
>>> Well this is completely unfixable. See the whole purpose of TTM is to
>>> allow tracing where what is mapped of a buffer object.
>>>
>>> If you circumvent that and increase the page reference yourself than
>>> that whole functionality can't work correctly any more.
>> Are you suggesting that the problem is that TTM doesn't see the KVM page
>> faults/mappings?
> 
> Yes, and no. It's one of the issues, but there is more behind that (e.g.
> what happens when TTM switches from pages to local memory for backing a
> BO).

If KVM page fault could reach TTM, then it should be able to relocate
BO. I see now where is the problem, thanks. Although, I'm wondering
whether it already works somehow.. I'll try to play with the the AMDGPU
shrinker and see what will happen on guest mapping of a relocated BO.

> Another question is why is KVM accessing the page structure in the first
> place? The VMA is mapped with VM_PFNMAP and VM_IO, KVM should never ever
> touch any of those pages.

https://elixir.bootlin.com/linux/v5.19/source/virt/kvm/kvm_main.c#L2549

-- 
Best regards,
Dmitry