Re: [PATCH v4 01/14] drm/ttm: Remap all page faults to per process dummy page.

[Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>]

On 1/19/21 8:56 AM, Daniel Vetter wrote:
> On Mon, Jan 18, 2021 at 04:01:10PM -0500, Andrey Grodzovsky wrote:
>> On device removal reroute all CPU mappings to dummy page.
>>
>> v3:
>> Remove loop to find DRM file and instead access it
>> by vma->vm_file->private_data. Move dummy page installation
>> into a separate function.
>>
>> v4:
>> Map the entire BOs VA space into on demand allocated dummy page
>> on the first fault for that BO.
>>
>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>> ---
>>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 82 ++++++++++++++++++++++++++++++++++++++++-
>>   include/drm/ttm/ttm_bo_api.h    |  2 +
>>   2 files changed, 83 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> index 6dc96cf..ed89da3 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> @@ -34,6 +34,8 @@
>>   #include <drm/ttm/ttm_bo_driver.h>
>>   #include <drm/ttm/ttm_placement.h>
>>   #include <drm/drm_vma_manager.h>
>> +#include <drm/drm_drv.h>
>> +#include <drm/drm_managed.h>
>>   #include <linux/mm.h>
>>   #include <linux/pfn_t.h>
>>   #include <linux/rbtree.h>
>> @@ -380,25 +382,103 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>>   }
>>   EXPORT_SYMBOL(ttm_bo_vm_fault_reserved);
>>   
>> +static void ttm_bo_release_dummy_page(struct drm_device *dev, void *res)
>> +{
>> +	struct page *dummy_page = (struct page *)res;
>> +
>> +	__free_page(dummy_page);
>> +}
>> +
>> +vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot)
>> +{
>> +	struct vm_area_struct *vma = vmf->vma;
>> +	struct ttm_buffer_object *bo = vma->vm_private_data;
>> +	struct ttm_bo_device *bdev = bo->bdev;
>> +	struct drm_device *ddev = bo->base.dev;
>> +	vm_fault_t ret = VM_FAULT_NOPAGE;
>> +	unsigned long address = vma->vm_start;
>> +	unsigned long num_prefault = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
>> +	unsigned long pfn;
>> +	struct page *page;
>> +	int i;
>> +
>> +	/*
>> +	 * Wait for buffer data in transit, due to a pipelined
>> +	 * move.
>> +	 */
>> +	ret = ttm_bo_vm_fault_idle(bo, vmf);
>> +	if (unlikely(ret != 0))
>> +		return ret;
>> +
>> +	/* Allocate new dummy page to map all the VA range in this VMA to it*/
>> +	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
>> +	if (!page)
>> +		return VM_FAULT_OOM;
>> +
>> +	pfn = page_to_pfn(page);
>> +
>> +	/*
>> +	 * Prefault the entire VMA range right away to avoid further faults
>> +	 */
>> +	for (i = 0; i < num_prefault; ++i) {
>> +
>> +		if (unlikely(address >= vma->vm_end))
>> +			break;
>> +
>> +		if (vma->vm_flags & VM_MIXEDMAP)
>> +			ret = vmf_insert_mixed_prot(vma, address,
>> +						    __pfn_to_pfn_t(pfn, PFN_DEV),
>> +						    prot);
>> +		else
>> +			ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>> +
>> +		/* Never error on prefaulted PTEs */
>> +		if (unlikely((ret & VM_FAULT_ERROR))) {
>> +			if (i == 0)
>> +				return VM_FAULT_NOPAGE;
>> +			else
>> +				break;
>> +		}
>> +
>> +		address += PAGE_SIZE;
>> +	}
>> +
>> +	/* Set the page to be freed using drmm release action */
>> +	if (drmm_add_action_or_reset(ddev, ttm_bo_release_dummy_page, page))
>> +		return VM_FAULT_OOM;
>> +
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL(ttm_bo_vm_dummy_page);
> I think we can lift this entire thing (once the ttm_bo_vm_fault_idle is
> gone) to the drm level, since nothing ttm specific in here. Probably stuff
> it into drm_gem.c (but really it's not even gem specific, it's fully
> generic "replace this vma with dummy pages pls" function.


Once I started with this I noticed that drmm_add_action_or_reset depends
on struct drm_device *ddev = bo->base.dev  and bo is the private data
we embed at the TTM level when setting up the mapping and so this forces
to move drmm_add_action_or_reset out of this function to every client who uses
this function, and then you separate the logic of page allocation from it's release.
So I suggest we keep it as is.

Andrey


>
> Aside from this nit I think the overall approach you have here is starting
> to look good. Lots of work&polish, but imo we're getting there and can
> start landing stuff soon.
> -Daniel
>
>> +
>>   vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
>>   {
>>   	struct vm_area_struct *vma = vmf->vma;
>>   	pgprot_t prot;
>>   	struct ttm_buffer_object *bo = vma->vm_private_data;
>> +	struct drm_device *ddev = bo->base.dev;
>>   	vm_fault_t ret;
>> +	int idx;
>>   
>>   	ret = ttm_bo_vm_reserve(bo, vmf);
>>   	if (ret)
>>   		return ret;
>>   
>>   	prot = vma->vm_page_prot;
>> -	ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT, 1);
>> +	if (drm_dev_enter(ddev, &idx)) {
>> +		ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT, 1);
>> +		drm_dev_exit(idx);
>> +	} else {
>> +		ret = ttm_bo_vm_dummy_page(vmf, prot);
>> +	}
>>   	if (ret == VM_FAULT_RETRY && !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
>>   		return ret;
>>   
>>   	dma_resv_unlock(bo->base.resv);
>>   
>>   	return ret;
>> +
>> +	return ret;
>>   }
>>   EXPORT_SYMBOL(ttm_bo_vm_fault);
>>   
>> diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
>> index e17be32..12fb240 100644
>> --- a/include/drm/ttm/ttm_bo_api.h
>> +++ b/include/drm/ttm/ttm_bo_api.h
>> @@ -643,4 +643,6 @@ void ttm_bo_vm_close(struct vm_area_struct *vma);
>>   int ttm_bo_vm_access(struct vm_area_struct *vma, unsigned long addr,
>>   		     void *buf, int len, int write);
>>   
>> +vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot);
>> +
>>   #endif
>> -- 
>> 2.7.4
>>
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