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

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Message-ID: <86a87de8-24a9-3c53-3ac7-612ca97e41df@collabora.com>
Date:   Mon, 15 Aug 2022 14:19:28 +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 13:51, Christian König wrote:
> Am 15.08.22 um 12:47 schrieb Dmitry Osipenko:
>> On 8/15/22 13:18, Dmitry Osipenko wrote:
>>> On 8/15/22 13:14, Christian König wrote:
>>>> Am 15.08.22 um 12:11 schrieb Christian König:
>>>>> Am 15.08.22 um 12:09 schrieb Dmitry Osipenko:
>>>>>> On 8/15/22 13:05, Christian König wrote:
>>>>>>> Am 15.08.22 um 11:54 schrieb Dmitry Osipenko:
>>>>>>>> Higher order pages allocated using alloc_pages() aren't
>>>>>>>> refcounted and
>>>>>>>> they
>>>>>>>> need to be refcounted, otherwise it's impossible to map them by
>>>>>>>> KVM. This
>>>>>>>> patch sets the refcount of the tail pages and fixes the KVM memory
>>>>>>>> mapping
>>>>>>>> faults.
>>>>>>>>
>>>>>>>> Without this change guest virgl driver can't map host buffers into
>>>>>>>> guest
>>>>>>>> and can't provide OpenGL 4.5 profile support to the guest. The host
>>>>>>>> mappings are also needed for enabling the Venus driver using
>>>>>>>> host GPU
>>>>>>>> drivers that are utilizing TTM.
>>>>>>>>
>>>>>>>> Based on a patch proposed by Trigger Huang.
>>>>>>> Well I can't count how often I have repeated this: This is an
>>>>>>> absolutely
>>>>>>> clear NAK!
>>>>>>>
>>>>>>> TTM pages are not reference counted in the first place and
>>>>>>> because of
>>>>>>> this giving them to virgl is illegal.
>>>>>> A? The first page is refcounted when allocated, the tail pages are
>>>>>> not.
>>>>> No they aren't. The first page is just by coincident initialized with
>>>>> a refcount of 1. This refcount is completely ignored and not used
>>>>> at all.
>>>>>
>>>>> Incrementing the reference count and by this mapping the page into
>>>>> some other address space is illegal and corrupts the internal state
>>>>> tracking of TTM.
>>>> See this comment in the source code as well:
>>>>
>>>>          /* Don't set the __GFP_COMP flag for higher order allocations.
>>>>           * Mapping pages directly into an userspace process and
>>>> calling
>>>>           * put_page() on a TTM allocated page is illegal.
>>>>           */
>>>>
>>>> I have absolutely no idea how somebody had the idea he could do this.
>>> I saw this comment, but it doesn't make sense because it doesn't explain
>>> why it's illegal. Hence it looks like a bogus comment since the
>>> refcouting certainly works, at least to a some degree because I haven't
>>> noticed any problems in practice, maybe by luck :)
>>>
>>> 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 :/

>> 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?

-- 
Best regards,
Dmitry