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

From: "Christian König via Virtualization" <virtualization@lists.linux-foundation.org>
To: Dmitry Osipenko <dmitry.osipenko@collabora.com>,
	David Airlie <airlied@linux.ie>, Huang Rui <ray.huang@amd.com>,
	Daniel Vetter <daniel@ffwll.ch>,
	Trigger Huang <Trigger.Huang@gmail.com>,
	Gert Wollny <gert.wollny@collabora.com>,
	Antonio Caggiano <antonio.caggiano@collabora.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org,
	virtualization@lists.linux-foundation.org,
	Dmitry Osipenko <digetx@gmail.com>,
	kernel@collabora.com
Subject: Re: [PATCH v1] drm/ttm: Refcount allocated tail pages
Date: Mon, 15 Aug 2022 13:28:11 +0200	[thread overview]
Message-ID: <8f749cd0-9a04-7c72-6a4f-a42d501e1489@amd.com> (raw)
In-Reply-To: <86a87de8-24a9-3c53-3ac7-612ca97e41df@collabora.com>

Am 15.08.22 um 13:19 schrieb Dmitry Osipenko:
> [SNIP]
>>>> I'll try to dig out the older discussions, thank you for the quick
>>>> reply!
>>> Are you sure it was really discussed in public previously? All I can
>>> find is yours two answers to a similar patches where you're saying that
>>> this it's a wrong solution without in-depth explanation and further
>>> discussions.
>> Yeah, that's my problem as well I can't find that of hand.
>>
>> But yes it certainly was discussed in public.
> If it was only CC'd to dri-devel, then could be that emails didn't pass
> the spam moderation :/

That might be possible.

>>> 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).

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.

Regards,
Christian.
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