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

[Aneesh Kumar K . V]

Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> writes:

> On 12.01.2026 13:22, Aneesh Kumar K.V wrote:
>> Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> writes:
>>> On 09.01.2026 04:15, Aneesh Kumar K.V wrote:
>>>> Robin Murphy <robin.murphy@xxxxxxx> 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 ?

-aneesh