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 K?nig wrote:
>>> Am 15.08.22 um 12:11 schrieb Christian K?nig:
>>>> Am 15.08.22 um 12:09 schrieb Dmitry Osipenko:
>>>>> On 8/15/22 13:05, Christian K?nig 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.
>
> 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.
Regards,
Christian.