Re: [PATCH v2 1/8] drm: Add dummy page per device or GEM object

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

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@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@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@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&amp;data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7C00053e9d983041ed63ae08d88882ed87%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637409443224016377%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=kghiG3VpCJod6YefExoDVPl9X03zNhw3SN5GAxgbnmU%3D&amp;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&amp;data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7C00053e9d983041ed63ae08d88882ed87%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637409443224016377%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=ziHJtqyHuLrlb0uYKhoWCWhUAZnX0JquE%2BkBJ5Fx%2BNo%3D&amp;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&amp;data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7C00053e9d983041ed63ae08d88882ed87%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637409443224016377%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=h360c75Upl3%2FW7im7M1%2BxY%2FXy4gxin%2BkCF1Ui2zFXMs%3D&amp;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&amp;data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7C00053e9d983041ed63ae08d88882ed87%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637409443224021379%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=sx6s1lH%2FvxbIZajc4Yr49vFhxvPEnBHZlTt52D8qvZA%3D&amp;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.

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
>>

_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel