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Date:   Mon, 15 Aug 2022 13:47:01 +0300
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Subject: Re: [PATCH v1] drm/ttm: Refcount allocated tail pages
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From:   Dmitry Osipenko <dmitry.osipenko@collabora.com>
To:     =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.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:     dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Dmitry Osipenko <digetx@gmail.com>, kvm@vger.kernel.org,
        kernel@collabora.com, virtualization@lists.linux-foundation.org
References: <20220815095423.11131-1-dmitry.osipenko@collabora.com>
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 <134bce02-58d6-8553-bb73-42dfda18a595@collabora.com>
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 <4fcc4739-2da9-1b89-209c-876129604d7d@amd.com>
 <14be3b22-1d60-732b-c695-ddacc6b21055@collabora.com>
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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.

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.

-- 
Best regards,
Dmitry