Re: [PATCH] drm/ttm: use compound pages for THP

["Christian König" <christian.koenig@amd.com>]

Am 13.01.21 um 10:33 schrieb David Stevens:
> On Wed, Jan 13, 2021 at 5:58 PM Christian König
> <christian.koenig@amd.com> wrote:
>> Am 13.01.21 um 09:47 schrieb David Stevens:
>>> Compound page metadata is necessary for page reference counting to work
>>> properly on pages besides the head page. Without it, put_page
>>> corresponding to the last outstanding get_page call on a tail page will
>>> end up freeing that page, even if the bo still references the page.
>> NAK, I should add a comment since I can't count any more how many times
>> people came up with this.
>>
>> Calling put_page() on a TTM allocated page is strictly illegal in the
>> first place since they are not reference counted.
>>
>> Please don't tell me somebody once more tried to mmap() pages from a
>> DMA-buf created sg_table into a process address space?
> I ran into this on a system using the currently in development
> virtio-gpu blob resources [1]. The implementation passes dma buffers
> allocated by the host gpu to KVM_SET_USER_MEMORY_REGION, so the guest
> can directly access the buffers without the need for an intermediate
> copy.

Yeah, that is completely illegal. They risk memory corruption with that.

You should probably report back to the author of those patches and not 
that the dma_buf_mmap() functions need to be used for this use case.

Regards,
Christian.

>
> [1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatchwork.kernel.org%2Fproject%2Fdri-devel%2Fcover%2F20200814024000.2485-1-gurchetansingh%40chromium.org%2F&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C4d93e46264e54f7d047908d8b7a64991%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637461272261264257%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=XH4UM65P6tNwJ4CUxt12TmBemEgl6q8gQYY0EqdBtyc%3D&amp;reserved=0
>
>> Regards,
>> Christian.
>>
>>> Signed-off-by: David Stevens <stevensd@chromium.org>
>>> ---
>>>    drivers/gpu/drm/ttm/ttm_pool.c | 1 -
>>>    1 file changed, 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
>>> index 7b2f60616750..09239b93dc2c 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_pool.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_pool.c
>>> @@ -83,7 +83,6 @@ static struct page *ttm_pool_alloc_page(struct ttm_pool *pool, gfp_t gfp_flags,
>>>                gfp_flags |= GFP_TRANSHUGE_LIGHT | __GFP_NORETRY |
>>>                        __GFP_KSWAPD_RECLAIM;
>>>                gfp_flags &= ~__GFP_MOVABLE;
>>> -             gfp_flags &= ~__GFP_COMP;
>>>        }
>>>
>>>        if (!pool->use_dma_alloc) {

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