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

From: Aneesh Kumar K.V <[email-masked]>
Date: Thu, 14 May 2026 10:24:48 +0530

Mostafa Saleh <[email-masked]> writes:

> 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]/
>

I was not sure whether we need dma_page. As shown in this series, we can
simplify the allocation and free paths without adding new abstractions
like dma_page.

>
>>  			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().
>

The reason I added that HighMem check is that __dma_direct_alloc_pages()
already has that check.

	page = dma_alloc_contiguous(dev, size, gfp);
	if (page) {
		if (dma_coherent_ok(dev, page_to_phys(page), size) &&
		    (allow_highmem || !PageHighMem(page)))
			return page;

		dma_free_contiguous(dev, page, size);
	}

I understand that the current usage of swiotlb alloc is restricted to
restricted memory, and it will not return HighMem pages. I will drop
this hunk from the patch.

-aneesh