Re: [PATCH] ARM: dma-mapping: split cache maintenance at highmem boundary

[Arnd Bergmann]

On Wed, Sep 9, 2026, at 08:44, Karl Mehltretter wrote:
> dma_cache_maint_page() processes highmem pages one at a time, but passes
> the complete remaining range to a cache operation when the current page
> is lowmem. If a physically contiguous range starts in lowmem and crosses
> into highmem, the operation continues beyond high_memory through virtual
> addresses which do not map the highmem pages.
>
> On a Raspberry Pi 400 running an ARM32 LPAE kernel, an 8 KiB scatterlist
> entry crossing this boundary caused a deterministic data abort at f0000000
> in v7_dma_clean_range().
>
> Limit the crossing lowmem iteration to the end of its page. Subsequent
> iterations advance page by page until the existing highmem path takes
> over. Keep the bulk operation for ranges contained in lowmem.
>
> Fixes: 43377453af83 ("[ARM] introduce dma_cache_maint_page()")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>

Hi Karl,

I can see how the fix addresses the problem, but I don't yet see
how you can arrive in this situation. What type of memory allocation
can produce a physically contiguous page range from multiple
zones?

      Arnd