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

[Christian König]

Am 18.01.21 um 22:01 schrieb Andrey Grodzovsky:
> 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 at 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;

This is superfluous and probably quite harmful here because we wait for 
the hardware to do something.

We map a dummy page instead of the real BO content to the whole range 
anyway, so no need to wait for the real BO content to show up.

> +
> +	/* 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) {

Maybe rename the variable to num_pages. I was confused for a moment why 
we still prefault.

Alternative you can just drop i and do "for (addr = vma->vm_start; addr 
< vma->vm_end; addr += PAGE_SIZE)".

> +
> +		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;

This should probably be modified to either always return the error or 
always ignore it.

Apart from that looks good to me.

Christian.

> +		}
> +
> +		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);
> +
>   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