[PATCH v1 3/5] drm/pagemap: DMA map folios when possible

[Matthew Brost]

On Thu, Jul 17, 2025 at 03:38:25PM +0200, Francois Dugast wrote:
> If the page is part of a folio, DMA map the whole folio at once instead of
> mapping individual pages one after the other. For example if 2MB folios
> are used instead of 4KB pages, this reduces the number of DMA mappings by
> 512.
> 
> The folio order (and consequently, the size) is persisted in the struct
> drm_pagemap_device_addr to be available at the time of unmapping.
> 
> Signed-off-by: Francois Dugast <francois.dugast at intel.com>
> Cc: Matthew Brost <matthew.brost at intel.com>
> ---
>  drivers/gpu/drm/drm_pagemap.c | 28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
> index 0ed66aaade14..de15d96f6393 100644
> --- a/drivers/gpu/drm/drm_pagemap.c
> +++ b/drivers/gpu/drm/drm_pagemap.c
> @@ -220,20 +220,31 @@ static int drm_pagemap_migrate_map_pages(struct device *dev,
>  					 unsigned long npages,
>  					 enum dma_data_direction dir)
>  {
> -	unsigned long i;
> +	unsigned long i, j;
>  
> -	for (i = 0; i < npages; ++i) {
> +	for (i = 0; i < npages;) {
>  		struct page *page = migrate_pfn_to_page(migrate_pfn[i]);
> +		struct folio *folio;
> +		unsigned int order;

order is unitialized and if page is NULL, loop increment (next lable)
behavior will be undefined.

Matt

>  
>  		if (!page)
> -			continue;
> +			goto next;
>  
>  		if (WARN_ON_ONCE(is_zone_device_page(page)))
>  			return -EFAULT;
>  
> -		device_addr[i].addr = dma_map_page(dev, page, 0, PAGE_SIZE, dir);
> +		folio = page_folio(page);
> +		order = folio_order(folio);
> +		device_addr[i].addr = dma_map_page(dev, page, 0, page_size(page), dir);
> +		device_addr[i].order = order;
> +
>  		if (dma_mapping_error(dev, device_addr[i].addr))
>  			return -EFAULT;
> +
> +		for (j = 0; j < (0x1 << order) && i+j < npages; j++)
> +			device_addr[i+j].addr = device_addr[i].addr + j * PAGE_SIZE;
> +next:
> +		i += 0x1 << order;
>  	}
>  
>  	return 0;
> @@ -257,11 +268,14 @@ static void drm_pagemap_migrate_unmap_pages(struct device *dev,
>  {
>  	unsigned long i;
>  
> -	for (i = 0; i < npages; ++i) {
> +	for (i = 0; i < npages;) {
>  		if (!device_addr[i].addr || dma_mapping_error(dev, device_addr[i].addr))
> -			continue;
> +			goto next;
> +
> +		dma_unmap_page(dev, device_addr[i].addr, PAGE_SIZE << device_addr[i].order, dir);
>  
> -		dma_unmap_page(dev, device_addr[i].addr, PAGE_SIZE, dir);
> +next:
> +		i += 0x1 << device_addr[i].order;
>  	}
>  }
>  
> -- 
> 2.43.0
>