linux-kernel - Re: [PATCH] dma-direct: Skip cache prep for HighMem coherent allocations

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Message-ID: <87dc0a06-5818-45f3-99c9-d1aef807873f@samsung.com>
Date: Mon, 19 Jan 2026 10:18:13 +0100
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>, Robin Murphy
	<robin.murphy@....com>, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: Arnd Bergmann <arnd@...nel.org>, Linus Walleij <linusw@...nel.org>,
	Matthew Wilcox <willy@...radead.org>, Suzuki K Poulose
	<suzuki.poulose@....com>
Subject: Re: [PATCH] dma-direct: Skip cache prep for HighMem coherent
 allocations

On 19.01.2026 05:13, Aneesh Kumar K.V wrote:
> Marek Szyprowski <m.szyprowski@...sung.com> writes:
>> On 12.01.2026 13:22, Aneesh Kumar K.V wrote:
>>> Marek Szyprowski <m.szyprowski@...sung.com> writes:
>>>> On 09.01.2026 04:15, Aneesh Kumar K.V wrote:
>>>>> Robin Murphy <robin.murphy@....com> writes:
>>>>>> On 2026-01-08 12:41 pm, Marek Szyprowski wrote:
>>>>>>> On 08.01.2026 11:50, Robin Murphy wrote:
>>>>>>>> On 2026-01-02 3:51 pm, Aneesh Kumar K.V (Arm) wrote:
>>>>>>>>> dma_direct_alloc() calls arch_dma_prep_coherent() to clean any dirty
>>>>>>>>> cache lines from the kernel linear alias before creating a coherent
>>>>>>>>> remapping.
>>>>>>>>>
>>>>>>>>> HighMem pages have no kernel alias mapping, so there are no alias cache
>>>>>>>>> lines to clean. Skip arch_dma_prep_coherent() for HighMem allocations.
>>>>>>>> This is assuming that caches are always cleaned when unmapping
>>>>>>>> highmem, and no still-mapped highmem pages are dirty - how is that
>>>>>>>> guaranteed? The fact that they're not in the linear map doesn't mean
>>>>>>>> they don't necessarily have kernel aliases in either vmalloc
>>>>>>>> pagetables or caches.
>>>>>>> Right, so it is better to keep this unconditional
>>>>>>> arch_dma_prep_coherent() call. I will drop it from dma-mapping-fixes then.
>>>>>> Yeah, I think the confusing thing here is that there are architectures
>>>>>> with CONFIG_HIGHMEM that don't actually check for and handle it in their
>>>>>> arch_dma_prep_coherent() as they seemingly should, however I'm not sure
>>>>>> off-hand whether they also support/use highmem CMA in the manner that
>>>>>> could end up being an issue in practice (the lack of any reports of
>>>>>> crashes or DMA corruption over the last however many years suggests not...)
>>>>>>
>>>>> Should we then remove the PageHighMem() check with DMA_ATTR_NO_KERNEL_MAPPING?
>>>> Right, this has to be unified.
>>> I had a related question, how do we handle cache flushes required for
>>> architectures that don't implement CONFIG_ARCH_HAS_DMA_PREP_COHERENT (arch/arm)?
>> ARM 32bit architecture provides arch_dma_alloc(), which handles cache
>> management internally.
>>
> But dma_direct_alloc does the below
>
> 	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> 	    !force_dma_unencrypted(dev) && !is_swiotlb_for_alloc(dev))
> 		return dma_direct_alloc_no_mapping(dev, size, dma_handle, gfp);
>
> 	if (!dev_is_dma_coherent(dev)) {
> 		if (IS_ENABLED(CONFIG_ARCH_HAS_DMA_ALLOC) &&
> 		    !is_swiotlb_for_alloc(dev))
> 			return arch_dma_alloc(dev, size, dma_handle, gfp,
> 					      attrs);
>
> IIUC, this implies we won't call arch_dma_alloc for
> DMA_ATTR_NO_KERNEL_MAPPING ?

It looks that commit 849facea92fa ("dma-direct: simplify the 
DMA_ATTR_NO_KERNEL_MAPPING handling") changed this. Before that 
arch_dma_alloc() was called regardless of the provided DMA attrs. 
Indeed, this should be fixed. Frankly speaking this probably means that 
there are no active users of this combination (arch/arm, 
DMA_ATTR_NO_KERNEL_MAPPING and dma-direct) or nobody cares.

The DMA_ATTR_NO_KERNEL_MAPPING attribute was initially introduced for 
the DMA-IOMMU based implementation, where it saves significant amount of 
resources.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland