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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Robin Murphy <robin.murphy@arm.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?

-aneesh