Re: [PATCH v6 4/5] drm/pagemap: Use dma-map IOVA alloc, link, and sync API for DRM pagemap

[Francois Dugast <francois.dugast@intel.com>]

On Wed, Apr 08, 2026 at 01:15:36PM -0700, Matthew Brost wrote:
> The dma-map IOVA alloc, link, and sync APIs perform significantly better
> than dma-map / dma-unmap, as they avoid costly IOMMU synchronizations.
> This difference is especially noticeable when mapping a 2MB region in
> 4KB pages.
> 
> Use the IOVA alloc, link, and sync APIs for DRM pagemap, which create DMA
> mappings between the CPU and GPU for copying data.
> 
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>

I have been running more tests this week: this improves SVM page fault
handling latency by around 4% for 64kB and by around 20% for 2MB, nice!

Reviewed-by: Francois Dugast <francois.dugast@intel.com>

Francois

> 
> ---
> v6: Fix drm_pagemap_migrate_map_system_pages kernel doc (Francois)
> ---
>  drivers/gpu/drm/drm_pagemap.c | 85 ++++++++++++++++++++++++++++-------
>  1 file changed, 70 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
> index ee4d9f90bf67..ed62866b52f6 100644
> --- a/drivers/gpu/drm/drm_pagemap.c
> +++ b/drivers/gpu/drm/drm_pagemap.c
> @@ -291,6 +291,19 @@ drm_pagemap_migrate_map_device_private_pages(struct device *dev,
>  	return 0;
>  }
>  
> +/**
> + * struct drm_pagemap_iova_state - DRM pagemap IOVA state
> + * @dma_state: DMA IOVA state.
> + * @offset: Current offset in IOVA.
> + *
> + * This structure acts as an iterator for packing all IOVA addresses within a
> + * contiguous range.
> + */
> +struct drm_pagemap_iova_state {
> +	struct dma_iova_state dma_state;
> +	unsigned long offset;
> +};
> +
>  /**
>   * drm_pagemap_migrate_map_system_pages() - Map system or device coherent
>   * migration pages for GPU SVM migration
> @@ -299,22 +312,25 @@ drm_pagemap_migrate_map_device_private_pages(struct device *dev,
>   * @migrate_pfn: Array of page frame numbers of system pages or peer pages to map.
>   * @npages: Number of system or device coherent pages to map.
>   * @dir: Direction of data transfer (e.g., DMA_BIDIRECTIONAL)
> + * @state: DMA IOVA state for mapping.
>   *
>   * This function maps pages of memory for migration usage in GPU SVM. It
>   * iterates over each page frame number provided in @migrate_pfn, maps the
>   * corresponding page, and stores the DMA address in the provided @dma_addr
>   * array.
>   *
> - * Returns: 0 on success, -EFAULT if an error occurs during mapping.
> + * Returns: 0 on success, negative error code on failure.
>   */
>  static int
>  drm_pagemap_migrate_map_system_pages(struct device *dev,
>  				     struct drm_pagemap_addr *pagemap_addr,
>  				     unsigned long *migrate_pfn,
>  				     unsigned long npages,
> -				     enum dma_data_direction dir)
> +				     enum dma_data_direction dir,
> +				     struct drm_pagemap_iova_state *state)
>  {
>  	unsigned long i;
> +	bool try_alloc = false;
>  
>  	for (i = 0; i < npages;) {
>  		struct page *page = migrate_pfn_to_page(migrate_pfn[i]);
> @@ -329,9 +345,31 @@ drm_pagemap_migrate_map_system_pages(struct device *dev,
>  		folio = page_folio(page);
>  		order = folio_order(folio);
>  
> -		dma_addr = dma_map_page(dev, page, 0, page_size(page), dir);
> -		if (dma_mapping_error(dev, dma_addr))
> -			return -EFAULT;
> +		if (!try_alloc) {
> +			dma_iova_try_alloc(dev, &state->dma_state,
> +					   (npages - i) * PAGE_SIZE >=
> +					   HPAGE_PMD_SIZE ?
> +					   HPAGE_PMD_SIZE : 0,
> +					   npages * PAGE_SIZE);
> +			try_alloc = true;
> +		}
> +
> +		if (dma_use_iova(&state->dma_state)) {
> +			int err = dma_iova_link(dev, &state->dma_state,
> +						page_to_phys(page),
> +						state->offset, page_size(page),
> +						dir, 0);
> +			if (err)
> +				return err;
> +
> +			dma_addr = state->dma_state.addr + state->offset;
> +			state->offset += page_size(page);
> +		} else {
> +			dma_addr = dma_map_page(dev, page, 0, page_size(page),
> +						dir);
> +			if (dma_mapping_error(dev, dma_addr))
> +				return -EFAULT;
> +		}
>  
>  		pagemap_addr[i] =
>  			drm_pagemap_addr_encode(dma_addr,
> @@ -342,6 +380,9 @@ drm_pagemap_migrate_map_system_pages(struct device *dev,
>  		i += NR_PAGES(order);
>  	}
>  
> +	if (dma_use_iova(&state->dma_state))
> +		return dma_iova_sync(dev, &state->dma_state, 0, state->offset);
> +
>  	return 0;
>  }
>  
> @@ -353,6 +394,7 @@ drm_pagemap_migrate_map_system_pages(struct device *dev,
>   * @pagemap_addr: Array of DMA information corresponding to mapped pages
>   * @npages: Number of pages to unmap
>   * @dir: Direction of data transfer (e.g., DMA_BIDIRECTIONAL)
> + * @state: DMA IOVA state for mapping.
>   *
>   * This function unmaps previously mapped pages of memory for GPU Shared Virtual
>   * Memory (SVM). It iterates over each DMA address provided in @dma_addr, checks
> @@ -362,10 +404,17 @@ static void drm_pagemap_migrate_unmap_pages(struct device *dev,
>  					    struct drm_pagemap_addr *pagemap_addr,
>  					    unsigned long *migrate_pfn,
>  					    unsigned long npages,
> -					    enum dma_data_direction dir)
> +					    enum dma_data_direction dir,
> +					    struct drm_pagemap_iova_state *state)
>  {
>  	unsigned long i;
>  
> +	if (state && dma_use_iova(&state->dma_state)) {
> +		dma_iova_unlink(dev, &state->dma_state, 0, state->offset, dir, 0);
> +		dma_iova_free(dev, &state->dma_state);
> +		return;
> +	}
> +
>  	for (i = 0; i < npages;) {
>  		struct page *page = migrate_pfn_to_page(migrate_pfn[i]);
>  
> @@ -420,7 +469,7 @@ drm_pagemap_migrate_remote_to_local(struct drm_pagemap_devmem *devmem,
>  			       devmem->pre_migrate_fence);
>  out:
>  	drm_pagemap_migrate_unmap_pages(remote_device, pagemap_addr, local_pfns,
> -					npages, DMA_FROM_DEVICE);
> +					npages, DMA_FROM_DEVICE, NULL);
>  	return err;
>  }
>  
> @@ -430,11 +479,13 @@ drm_pagemap_migrate_sys_to_dev(struct drm_pagemap_devmem *devmem,
>  			       struct page *local_pages[],
>  			       struct drm_pagemap_addr pagemap_addr[],
>  			       unsigned long npages,
> -			       const struct drm_pagemap_devmem_ops *ops)
> +			       const struct drm_pagemap_devmem_ops *ops,
> +			       struct drm_pagemap_iova_state *state)
>  {
>  	int err = drm_pagemap_migrate_map_system_pages(devmem->dev,
>  						       pagemap_addr, sys_pfns,
> -						       npages, DMA_TO_DEVICE);
> +						       npages, DMA_TO_DEVICE,
> +						       state);
>  
>  	if (err)
>  		goto out;
> @@ -443,7 +494,7 @@ drm_pagemap_migrate_sys_to_dev(struct drm_pagemap_devmem *devmem,
>  				  devmem->pre_migrate_fence);
>  out:
>  	drm_pagemap_migrate_unmap_pages(devmem->dev, pagemap_addr, sys_pfns, npages,
> -					DMA_TO_DEVICE);
> +					DMA_TO_DEVICE, state);
>  	return err;
>  }
>  
> @@ -471,6 +522,7 @@ static int drm_pagemap_migrate_range(struct drm_pagemap_devmem *devmem,
>  				     const struct migrate_range_loc *cur,
>  				     const struct drm_pagemap_migrate_details *mdetails)
>  {
> +	struct drm_pagemap_iova_state state = {};
>  	int ret = 0;
>  
>  	if (cur->start == 0)
> @@ -498,7 +550,7 @@ static int drm_pagemap_migrate_range(struct drm_pagemap_devmem *devmem,
>  						     &pages[last->start],
>  						     &pagemap_addr[last->start],
>  						     cur->start - last->start,
> -						     last->ops);
> +						     last->ops, &state);
>  
>  out:
>  	*last = *cur;
> @@ -1060,6 +1112,7 @@ EXPORT_SYMBOL(drm_pagemap_put);
>  int drm_pagemap_evict_to_ram(struct drm_pagemap_devmem *devmem_allocation)
>  {
>  	const struct drm_pagemap_devmem_ops *ops = devmem_allocation->ops;
> +	struct drm_pagemap_iova_state state = {};
>  	unsigned long npages, mpages = 0;
>  	struct page **pages;
>  	unsigned long *src, *dst;
> @@ -1101,7 +1154,7 @@ int drm_pagemap_evict_to_ram(struct drm_pagemap_devmem *devmem_allocation)
>  	err = drm_pagemap_migrate_map_system_pages(devmem_allocation->dev,
>  						   pagemap_addr,
>  						   dst, npages,
> -						   DMA_FROM_DEVICE);
> +						   DMA_FROM_DEVICE, &state);
>  	if (err)
>  		goto err_finalize;
>  
> @@ -1125,7 +1178,7 @@ int drm_pagemap_evict_to_ram(struct drm_pagemap_devmem *devmem_allocation)
>  	migrate_device_pages(src, dst, npages);
>  	migrate_device_finalize(src, dst, npages);
>  	drm_pagemap_migrate_unmap_pages(devmem_allocation->dev, pagemap_addr, dst, npages,
> -					DMA_FROM_DEVICE);
> +					DMA_FROM_DEVICE, &state);
>  
>  err_free:
>  	kvfree(buf);
> @@ -1170,6 +1223,7 @@ static int __drm_pagemap_migrate_to_ram(struct vm_area_struct *vas,
>  				  MIGRATE_VMA_SELECT_COMPOUND,
>  		.fault_page	= page,
>  	};
> +	struct drm_pagemap_iova_state state = {};
>  	struct drm_pagemap_zdd *zdd;
>  	const struct drm_pagemap_devmem_ops *ops;
>  	struct device *dev = NULL;
> @@ -1229,7 +1283,7 @@ static int __drm_pagemap_migrate_to_ram(struct vm_area_struct *vas,
>  
>  	err = drm_pagemap_migrate_map_system_pages(dev, pagemap_addr,
>  						   migrate.dst, npages,
> -						   DMA_FROM_DEVICE);
> +						   DMA_FROM_DEVICE, &state);
>  	if (err)
>  		goto err_finalize;
>  
> @@ -1254,7 +1308,8 @@ static int __drm_pagemap_migrate_to_ram(struct vm_area_struct *vas,
>  	migrate_vma_finalize(&migrate);
>  	if (dev)
>  		drm_pagemap_migrate_unmap_pages(dev, pagemap_addr, migrate.dst,
> -						npages, DMA_FROM_DEVICE);
> +						npages, DMA_FROM_DEVICE,
> +						&state);
>  err_free:
>  	kvfree(buf);
>  err_out:
> -- 
> 2.34.1
>