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

[Karl Mehltretter <kmehltretter@gmail.com>]

On Wed, Sep 09, 2026 at 09:24:23AM +0100, Arnd Bergmann wrote:
> 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?
> 

Hi Arnd,

You're right that a buddy allocation cannot span zones. I found this issue
during code review while investigating another ARM DMA issue. I plan to
post a patch for that soon.

My Pi test deliberately reserved the last lowmem and first highmem pages
in DT. It confirms the crash with a crossing SG entry, but I have not
found an ordinary workload that creates one. I should have made that
clear in the changelog.

In a follow-up QEMU test, sg_alloc_table_from_pages() merged the reserved
pair into one 8192-byte entry. It checks that the pages are physically
adjacent but does not check page_zone().

I also searched a bit for a real-world example. Rockchip's vendor 4.4
boot-logo code looks like a possible case: it builds an SG table from
firmware-reserved pages and calls dma_map_sg() when no display IOMMU is
used. Product logs show the boot-logo feature in use, but I found no
record of its reserved memory crossing the lowmem/highmem boundary.
Such a crossing seems unlikely.

I still think the change is worthwhile. For v2, I will update the
changelog to make the test setup and its limits clear. I may also drop the
Cc: stable tag.

Thanks,
Karl