Subject: Re: [PATCH v4 01/13] dma-direct: swiotlb: handle swiotlb alloc/free outside __dma_direct_alloc_pages

From: Mostafa Saleh <[email-masked]>
Date: Wed, 13 May 2026 13:57:16 +0000

On Tue, May 12, 2026 at 02:33:56PM +0530, Aneesh Kumar K.V (Arm) wrote:
> Move swiotlb allocation out of __dma_direct_alloc_pages() and handle it in
> dma_direct_alloc() / dma_direct_alloc_pages().
> 
> This is needed for follow-up changes that simplify the handling of
> memory encryption/decryption based on the DMA attribute flags.
> 
> swiotlb backing pages are already mapped decrypted by
> swiotlb_update_mem_attributes() and rmem_swiotlb_device_init(), so
> dma-direct should not call dma_set_decrypted() on allocation nor
> dma_set_encrypted() on free for swiotlb-backed memory.
> 
> Update alloc/free paths to detect swiotlb-backed pages and skip
> encrypt/decrypt transitions for those paths. Keep the existing highmem
> rejection in dma_direct_alloc_pages() for swiotlb allocations.
> 
> Only for "restricted-dma-pool", we currently set `for_alloc = true`, while
> rmem_swiotlb_device_init() decrypts the whole pool up front. This pool is
> typically used together with "shared-dma-pool", where the shared region is
> accessed after remap/ioremap and the returned address is suitable for
> decrypted memory access. So existing code paths remain valid.
> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <[email-masked]>
> ---
>  kernel/dma/direct.c | 44 +++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 37 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index ec887f443741..b958f150718a 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -125,9 +125,6 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
>  
>  	WARN_ON_ONCE(!PAGE_ALIGNED(size));
>  
> -	if (is_swiotlb_for_alloc(dev))
> -		return dma_direct_alloc_swiotlb(dev, size);
> -
>  	gfp |= dma_direct_optimal_gfp_mask(dev, &phys_limit);
>  	page = dma_alloc_contiguous(dev, size, gfp);
>  	if (page) {
> @@ -204,6 +201,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  		dma_addr_t *dma_handle, gfp_t gfp, unsigned long attrs)
>  {
>  	bool remap = false, set_uncached = false;
> +	bool mark_mem_decrypt = true;
>  	struct page *page;
>  	void *ret;
>  
> @@ -250,11 +248,21 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  	    dma_direct_use_pool(dev, gfp))
>  		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>  
> +	if (is_swiotlb_for_alloc(dev)) {
> +		page = dma_direct_alloc_swiotlb(dev, size);
> +		if (page) {
> +			mark_mem_decrypt = false;
> +			goto setup_page;
> +		}
> +		return NULL;
> +	}
> +
>  	/* we always manually zero the memory once we are done */
>  	page = __dma_direct_alloc_pages(dev, size, gfp & ~__GFP_ZERO, true);
>  	if (!page)
>  		return NULL;
>  
> +setup_page:
>  	/*
>  	 * dma_alloc_contiguous can return highmem pages depending on a
>  	 * combination the cma= arguments and per-arch setup.  These need to be
> @@ -281,7 +289,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  			goto out_free_pages;
>  	} else {
>  		ret = page_address(page);
> -		if (dma_set_decrypted(dev, ret, size))
> +		if (mark_mem_decrypt && dma_set_decrypted(dev, ret, size))

I am ok with that approach, but Jason was mentioning we shouldn’t
special case swiotlb and make the allocator return the memory state
(similar to the dma_page [1]) . I am also OK if you want to merge that
part of my series with is.

[1] https://lore.kernel.org/linux-iommu/[email-masked]/

>  			goto out_leak_pages;
>  	}
>  
> @@ -298,7 +306,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  	return ret;
>  
>  out_encrypt_pages:
> -	if (dma_set_encrypted(dev, page_address(page), size))
> +	if (mark_mem_decrypt && dma_set_encrypted(dev, page_address(page), size))
>  		return NULL;
>  out_free_pages:
>  	__dma_direct_free_pages(dev, page, size);
> @@ -310,6 +318,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  void dma_direct_free(struct device *dev, size_t size,
>  		void *cpu_addr, dma_addr_t dma_addr, unsigned long attrs)
>  {
> +	bool mark_mem_encrypted = true;
>  	unsigned int page_order = get_order(size);
>  
>  	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> @@ -338,12 +347,15 @@ void dma_direct_free(struct device *dev, size_t size,
>  	    dma_free_from_pool(dev, cpu_addr, PAGE_ALIGN(size)))
>  		return;
>  
> +	if (swiotlb_find_pool(dev, dma_to_phys(dev, dma_addr)))
> +		mark_mem_encrypted = false;
> +
>  	if (is_vmalloc_addr(cpu_addr)) {
>  		vunmap(cpu_addr);
>  	} else {
>  		if (IS_ENABLED(CONFIG_ARCH_HAS_DMA_CLEAR_UNCACHED))
>  			arch_dma_clear_uncached(cpu_addr, size);
> -		if (dma_set_encrypted(dev, cpu_addr, size))
> +		if (mark_mem_encrypted && dma_set_encrypted(dev, cpu_addr, size))
>  			return;
>  	}
>  
> @@ -359,6 +371,19 @@ struct page *dma_direct_alloc_pages(struct device *dev, size_t size,
>  	if (force_dma_unencrypted(dev) && dma_direct_use_pool(dev, gfp))
>  		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>  
> +	if (is_swiotlb_for_alloc(dev)) {
> +		page = dma_direct_alloc_swiotlb(dev, size);
> +		if (!page)
> +			return NULL;
> +
> +		if (PageHighMem(page)) {

My understanding is that rmem_swiotlb_device_init() asserts that there
is no PageHighMem()? Also a similar check doesn’t exist in
dma_direct_alloc().

Thanks,
Mostafa

> +			swiotlb_free(dev, page, size);
> +			return NULL;
> +		}
> +		ret = page_address(page);
> +		goto setup_page;
> +	}
> +
>  	page = __dma_direct_alloc_pages(dev, size, gfp, false);
>  	if (!page)
>  		return NULL;
> @@ -366,6 +391,7 @@ struct page *dma_direct_alloc_pages(struct device *dev, size_t size,
>  	ret = page_address(page);
>  	if (dma_set_decrypted(dev, ret, size))
>  		goto out_leak_pages;
> +setup_page:
>  	memset(ret, 0, size);
>  	*dma_handle = phys_to_dma_direct(dev, page_to_phys(page));
>  	return page;
> @@ -378,13 +404,17 @@ void dma_direct_free_pages(struct device *dev, size_t size,
>  		enum dma_data_direction dir)
>  {
>  	void *vaddr = page_address(page);
> +	bool mark_mem_encrypted = true;
>  
>  	/* If cpu_addr is not from an atomic pool, dma_free_from_pool() fails */
>  	if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
>  	    dma_free_from_pool(dev, vaddr, size))
>  		return;
>  
> -	if (dma_set_encrypted(dev, vaddr, size))
> +	if (swiotlb_find_pool(dev, page_to_phys(page)))
> +		mark_mem_encrypted = false;
> +
> +	if (mark_mem_encrypted && dma_set_encrypted(dev, vaddr, size))
>  		return;
>  	__dma_direct_free_pages(dev, page, size);
>  }
> -- 
> 2.43.0
>