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

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Date:   Mon, 15 Aug 2022 13:18:10 +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 13:14, 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!

-- 
Best regards,
Dmitry