Am 15.08.22 um 12:18 schrieb Dmitry Osipenko:> 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 :)
Well exactly that's the problem. It does not work, you are just lucky :)
I will provide a patch to set the reference count to zero even for
non-compound pages. Maybe that will yield more backtrace to abusers of
this interface.
Regards,
Christian.
>
> I'll try to dig out the older discussions, thank you for the quick
reply!
>