[PATCH v2 1/8] drm: Add dummy page per device or GEM object
Andrey Grodzovsky
Andrey.Grodzovsky at amd.com
Mon Nov 16 20:42:58 UTC 2020
On 11/16/20 3:36 PM, Christian König wrote:
> Am 16.11.20 um 20:00 schrieb Andrey Grodzovsky:
>>
>> On 11/16/20 4:48 AM, Christian König wrote:
>>> Am 15.11.20 um 07:34 schrieb Andrey Grodzovsky:
>>>>
>>>> On 11/14/20 4:51 AM, Daniel Vetter wrote:
>>>>> On Sat, Nov 14, 2020 at 9:41 AM Christian König
>>>>> <ckoenig.leichtzumerken at gmail.com> wrote:
>>>>>> Am 13.11.20 um 21:52 schrieb Andrey Grodzovsky:
>>>>>>> On 6/22/20 1:50 PM, Daniel Vetter wrote:
>>>>>>>> On Mon, Jun 22, 2020 at 7:45 PM Christian König
>>>>>>>> <christian.koenig at amd.com> wrote:
>>>>>>>>> Am 22.06.20 um 16:32 schrieb Andrey Grodzovsky:
>>>>>>>>>> On 6/22/20 9:18 AM, Christian König wrote:
>>>>>>>>>>> Am 21.06.20 um 08:03 schrieb Andrey Grodzovsky:
>>>>>>>>>>>> Will be used to reroute CPU mapped BO's page faults once
>>>>>>>>>>>> device is removed.
>>>>>>>>>>>>
>>>>>>>>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>>>>>>>>>>>> ---
>>>>>>>>>>>> drivers/gpu/drm/drm_file.c | 8 ++++++++
>>>>>>>>>>>> drivers/gpu/drm/drm_prime.c | 10 ++++++++++
>>>>>>>>>>>> include/drm/drm_file.h | 2 ++
>>>>>>>>>>>> include/drm/drm_gem.h | 2 ++
>>>>>>>>>>>> 4 files changed, 22 insertions(+)
>>>>>>>>>>>>
>>>>>>>>>>>> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
>>>>>>>>>>>> index c4c704e..67c0770 100644
>>>>>>>>>>>> --- a/drivers/gpu/drm/drm_file.c
>>>>>>>>>>>> +++ b/drivers/gpu/drm/drm_file.c
>>>>>>>>>>>> @@ -188,6 +188,12 @@ struct drm_file *drm_file_alloc(struct
>>>>>>>>>>>> drm_minor *minor)
>>>>>>>>>>>> goto out_prime_destroy;
>>>>>>>>>>>> }
>>>>>>>>>>>> + file->dummy_page = alloc_page(GFP_KERNEL | __GFP_ZERO);
>>>>>>>>>>>> + if (!file->dummy_page) {
>>>>>>>>>>>> + ret = -ENOMEM;
>>>>>>>>>>>> + goto out_prime_destroy;
>>>>>>>>>>>> + }
>>>>>>>>>>>> +
>>>>>>>>>>>> return file;
>>>>>>>>>>>> out_prime_destroy:
>>>>>>>>>>>> @@ -284,6 +290,8 @@ void drm_file_free(struct drm_file *file)
>>>>>>>>>>>> if (dev->driver->postclose)
>>>>>>>>>>>> dev->driver->postclose(dev, file);
>>>>>>>>>>>> + __free_page(file->dummy_page);
>>>>>>>>>>>> +
>>>>>>>>>>>> drm_prime_destroy_file_private(&file->prime);
>>>>>>>>>>>> WARN_ON(!list_empty(&file->event_list));
>>>>>>>>>>>> diff --git a/drivers/gpu/drm/drm_prime.c
>>>>>>>>>>>> b/drivers/gpu/drm/drm_prime.c
>>>>>>>>>>>> index 1de2cde..c482e9c 100644
>>>>>>>>>>>> --- a/drivers/gpu/drm/drm_prime.c
>>>>>>>>>>>> +++ b/drivers/gpu/drm/drm_prime.c
>>>>>>>>>>>> @@ -335,6 +335,13 @@ int drm_gem_prime_fd_to_handle(struct
>>>>>>>>>>>> drm_device *dev,
>>>>>>>>>>>> ret = drm_prime_add_buf_handle(&file_priv->prime,
>>>>>>>>>>>> dma_buf, *handle);
>>>>>>>>>>>> +
>>>>>>>>>>>> + if (!ret) {
>>>>>>>>>>>> + obj->dummy_page = alloc_page(GFP_KERNEL | __GFP_ZERO);
>>>>>>>>>>>> + if (!obj->dummy_page)
>>>>>>>>>>>> + ret = -ENOMEM;
>>>>>>>>>>>> + }
>>>>>>>>>>>> +
>>>>>>>>>>> While the per file case still looks acceptable this is a clear NAK
>>>>>>>>>>> since it will massively increase the memory needed for a prime
>>>>>>>>>>> exported object.
>>>>>>>>>>>
>>>>>>>>>>> I think that this is quite overkill in the first place and for the
>>>>>>>>>>> hot unplug case we can just use the global dummy page as well.
>>>>>>>>>>>
>>>>>>>>>>> Christian.
>>>>>>>>>> Global dummy page is good for read access, what do you do on write
>>>>>>>>>> access ? My first approach was indeed to map at first global dummy
>>>>>>>>>> page as read only and mark the vma->vm_flags as !VM_SHARED assuming
>>>>>>>>>> that this would trigger Copy On Write flow in core mm
>>>>>>>>>> (https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Fv5.7-rc7%2Fsource%2Fmm%2Fmemory.c%23L3977&data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7C00053e9d983041ed63ae08d88882ed87%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637409443224016377%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=kghiG3VpCJod6YefExoDVPl9X03zNhw3SN5GAxgbnmU%3D&reserved=0)
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> on the next page fault to same address triggered by a write access but
>>>>>>>>>> then i realized a new COW page will be allocated for each such mapping
>>>>>>>>>> and this is much more wasteful then having a dedicated page per GEM
>>>>>>>>>> object.
>>>>>>>>> Yeah, but this is only for a very very small corner cases. What we need
>>>>>>>>> to prevent is increasing the memory usage during normal operation to
>>>>>>>>> much.
>>>>>>>>>
>>>>>>>>> Using memory during the unplug is completely unproblematic because we
>>>>>>>>> just released quite a bunch of it by releasing all those system memory
>>>>>>>>> buffers.
>>>>>>>>>
>>>>>>>>> And I'm pretty sure that COWed pages are correctly accounted towards
>>>>>>>>> the
>>>>>>>>> used memory of a process.
>>>>>>>>>
>>>>>>>>> So I think if that approach works as intended and the COW pages are
>>>>>>>>> released again on unmapping it would be the perfect solution to the
>>>>>>>>> problem.
>>>>>>>>>
>>>>>>>>> Daniel what do you think?
>>>>>>>> If COW works, sure sounds reasonable. And if we can make sure we
>>>>>>>> managed to drop all the system allocations (otherwise suddenly 2x
>>>>>>>> memory usage, worst case). But I have no idea whether we can
>>>>>>>> retroshoehorn that into an established vma, you might have fun stuff
>>>>>>>> like a mkwrite handler there (which I thought is the COW handler
>>>>>>>> thing, but really no idea).
>>>>>>>>
>>>>>>>> If we need to massively change stuff then I think rw dummy page,
>>>>>>>> allocated on first fault after hotunplug (maybe just make it one per
>>>>>>>> object, that's simplest) seems like the much safer option. Much less
>>>>>>>> code that can go wrong.
>>>>>>>> -Daniel
>>>>>>>
>>>>>>> Regarding COW, i was looking into how to properly implement it from
>>>>>>> within the fault handler (i.e. ttm_bo_vm_fault)
>>>>>>> and the main obstacle I hit is that of exclusive access to the
>>>>>>> vm_area_struct, i need to be able to modify
>>>>>>> vma->vm_flags (and vm_page_prot) to remove VM_SHARED bit so COW can
>>>>>>> be triggered on subsequent write access
>>>>>>> fault (here
>>>>>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Flatest%2Fsource%2Fmm%2Fmemory.c%23L4128&data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7C00053e9d983041ed63ae08d88882ed87%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637409443224016377%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=ziHJtqyHuLrlb0uYKhoWCWhUAZnX0JquE%2BkBJ5Fx%2BNo%3D&reserved=0)
>>>>>>>
>>>>>>> but core mm takes only read side mm_sem (here for example
>>>>>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Flatest%2Fsource%2Fdrivers%2Fiommu%2Famd%2Fiommu_v2.c%23L488&data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7C00053e9d983041ed63ae08d88882ed87%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637409443224016377%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=h360c75Upl3%2FW7im7M1%2BxY%2FXy4gxin%2BkCF1Ui2zFXMs%3D&reserved=0)
>>>>>>>
>>>>>>> and so I am not supposed to modify vm_area_struct in this case. I am
>>>>>>> not sure if it's legit to write lock tthe mm_sem from this point.
>>>>>>> I found some discussions about this here
>>>>>>> https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Flkml.iu.edu%2Fhypermail%2Flinux%2Fkernel%2F1909.1%2F02754.html&data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7C00053e9d983041ed63ae08d88882ed87%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637409443224021379%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=sx6s1lH%2FvxbIZajc4Yr49vFhxvPEnBHZlTt52D8qvZA%3D&reserved=0
>>>>>>> but it
>>>>>>> wasn't really clear to me
>>>>>>> what's the solution.
>>>>>>>
>>>>>>> In any case, seems to me that easier and more memory saving solution
>>>>>>> would be to just switch to per ttm bo dumy rw page that
>>>>>>> would be allocated on demand as you suggested here. This should also
>>>>>>> take care of imported BOs and flink cases.
>>>>>>> Then i can drop the per device FD and per GEM object FD dummy BO and
>>>>>>> the ugly loop i am using in patch 2 to match faulting BO to the right
>>>>>>> dummy page.
>>>>>>>
>>>>>>> Does this makes sense ?
>>>>>> I still don't see the information leak as much of a problem, but if
>>>>>> Daniel insists we should probably do this.
>>>>> Well amdgpu doesn't clear buffers by default, so indeed you guys are a
>>>>> lot more laissez-faire here. But in general we really don't do that
>>>>> kind of leaking. Iirc there's even radeonsi bugs because else clears,
>>>>> and radeonsi happily displays gunk :-)
>>>>>
>>>>>> But could we at least have only one page per client instead of per BO?
>>>>> I think you can do one page per file descriptor or something like
>>>>> that. But gets annoying with shared bo, especially with dma_buf_mmap
>>>>> forwarding.
>>>>> -Daniel
>>>>
>>>>
>>>> Christian - is your concern more with too much page allocations or with
>>>> extra pointer member
>>>> cluttering TTM BO struct ?
>>>
>>> Yes, that is one problem.
>>>
>>>> Because we can allocate the dummy page on demand only when
>>>> needed. It's just seems to me that keeping it per BO streamlines the code
>>>> as I don't need to
>>>> have different handling for local vs imported BOs.
>>>
>>> Why should you have a difference between local vs imported BOs?
>>
>>
>> For local BO seems like Daniel's suggestion to use
>> vm_area_struct->vm_file->private_data
>> should work as this points to drm_file. For imported BOs private_data will
>> point to dma_buf structure
>> since each imported BO is backed by a pseudo file (created in dma_buf_getfile).
>
> Oh, good point. But we could easily fix that now. That should make the mapping
> code less complex as well.
Can you clarify what fix u have in mind ? I assume it's not by altering
file->private_data to point
to something else as we need to retrieve dmabuf (e.g. dma_buf_mmap_internal)
Andrey
>
> Regards,
> Christian.
>
>> If so,where should we store the dummy RW BO in this case ? In current
>> implementation it's stored in drm_gem_object.
>>
>> P.S For FLINK case it seems to me the handling should be no different then
>> with local BO as the
>> FD used for mmap in this case is still the same one associated with the DRM
>> file.
>>
>> Andrey
>>
>>
>>>
>>> Christian.
>>>
>>>>
>>>> Andrey
>>>
>
More information about the dri-devel
mailing list