On Fri, Aug 27, 2021 at 12:40 AM Christoph Hellwig [off-list ref] wrote:

quoted

In a PC world that would be (for example) the legacy PCI space at
0xa0000-0xfffff, but I could easily imagine other platforms having
other situations.

So what would be the correct check for "this is not actually page backed
normal RAM"?

It would probably be interesting to have the arm people explain the
call chain for the warning that caused that revert, so we'd have a
very concrete example of the situation that goes wrong, but taking a
wild stab at it, the code might be something like

            /* Don't allow RAM to be mapped */
            if (WARN_ON_ONCE(phys_addr_is_ram(phys_addr)))
                    return DMA_MAPPING_ERROR;

and then having something like

  static inline bool phys_addr_is_ram(phys_addr_t phys_addr)
  {
        unsigned long pfn = PHYS_PFN(phys_addr);

        if (!pfn_valid(pfn))
                return false;
        return is_zero_pfn(pfn) || !PageReserved(pfn_to_page(pfn));
  }

might be close to right.

The ARM code actually uses that complex pfn_to_section_nr() and
memblock_is_memory() etc. That seems a bit of an overkill, since the
memblock code should have translated all that into being reserved.

But again, I don't actually know exactly what triggered the issue on
ARM, so the above is just my "this seems to be a more proper check"
suggestion.

Will?

                   Linus

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