[PATCH v2 1/8] drm: Add dummy page per device or GEM object
Christian König
ckoenig.leichtzumerken at gmail.com
Sat Nov 14 08:41:03 UTC 2020
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=02%7C01%7CAndrey.Grodzovsky%40amd.com%7C3753451d037544e7495408d816d4c4ee%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637284450384586120&sdata=ZpRaQgqA5K4jRfidOiedey0AleeYQ97WNUkGA29ERA0%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://elixir.bootlin.com/linux/latest/source/mm/memory.c#L4128)
> but core mm takes only read side mm_sem (here for example
> https://elixir.bootlin.com/linux/latest/source/drivers/iommu/amd/iommu_v2.c#L488)
> 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
> http://lkml.iu.edu/hypermail/linux/kernel/1909.1/02754.html 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.
But could we at least have only one page per client instead of per BO?
Thanks,
Christian.
>
> Andrey
>
>
>>
>>> Regards,
>>> Christian.
>>>
>>>> We can indeed optimize by allocating this dummy page on the first page
>>>> fault after device disconnect instead on GEM object creation.
>>>>
>>>> Andrey
>>>>
>>>>
>>>>>> mutex_unlock(&file_priv->prime.lock);
>>>>>> if (ret)
>>>>>> goto fail;
>>>>>> @@ -1006,6 +1013,9 @@ void drm_prime_gem_destroy(struct
>>>>>> drm_gem_object *obj, struct sg_table *sg)
>>>>>> dma_buf_unmap_attachment(attach, sg, DMA_BIDIRECTIONAL);
>>>>>> dma_buf = attach->dmabuf;
>>>>>> dma_buf_detach(attach->dmabuf, attach);
>>>>>> +
>>>>>> + __free_page(obj->dummy_page);
>>>>>> +
>>>>>> /* remove the reference */
>>>>>> dma_buf_put(dma_buf);
>>>>>> }
>>>>>> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
>>>>>> index 19df802..349a658 100644
>>>>>> --- a/include/drm/drm_file.h
>>>>>> +++ b/include/drm/drm_file.h
>>>>>> @@ -335,6 +335,8 @@ struct drm_file {
>>>>>> */
>>>>>> struct drm_prime_file_private prime;
>>>>>> + struct page *dummy_page;
>>>>>> +
>>>>>> /* private: */
>>>>>> #if IS_ENABLED(CONFIG_DRM_LEGACY)
>>>>>> unsigned long lock_count; /* DRI1 legacy lock count */
>>>>>> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
>>>>>> index 0b37506..47460d1 100644
>>>>>> --- a/include/drm/drm_gem.h
>>>>>> +++ b/include/drm/drm_gem.h
>>>>>> @@ -310,6 +310,8 @@ struct drm_gem_object {
>>>>>> *
>>>>>> */
>>>>>> const struct drm_gem_object_funcs *funcs;
>>>>>> +
>>>>>> + struct page *dummy_page;
>>>>>> };
>>>>>> /**
>>
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