[PATCH 2/3] arm64: reimplement page_is_ram() using memblock and UEFI memory map

[Ard Biesheuvel]

On 12 November 2015 at 17:03, Mark Rutland <mark.rutland at arm.com> wrote:
> On Thu, Nov 12, 2015 at 04:40:23PM +0100, Ard Biesheuvel wrote:
>> On 12 November 2015 at 16:31, Matt Fleming <matt at codeblueprint.co.uk> wrote:
>> > On Thu, 29 Oct, at 02:40:58PM, Ard Biesheuvel wrote:
>> >> This patch overrides the __weak default implementation of page_is_ram(),
>> >> which uses string comparisons to find entries called 'System RAM' in
>> >> /proc/iomem. Since we used the contents of memblock to create those entries
>> >> in the first place, let's use memblock directly.
>> >>
>> >> Also, since the UEFI memory map may describe regions backed by RAM that are
>> >> not in memblock (i.e., reserved regions that were removed from the linear
>> >> mapping), check the pfn against the UEFI memory map as well.
>> >>
>> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> >> ---
>> >>  arch/arm64/mm/mmu.c | 34 ++++++++++++++++++++
>> >>  1 file changed, 34 insertions(+)
>> >
>> > Am I correct in thinking that the purpose of this series is just to
>> > placate acpi_os_ioremap() on arm64, and its use of page_is_ram()?
>> >
>>
>> That is currently the primary user, but we need this information for
>> other purposes as well. One example is /dev/mem, which is used for
>> both devices and memory (for instance, tools like dmidecode rely
>> heavily on it). When using it to access a memory region that we
>> punched out of the linear mapping, we should typically not map it as a
>> device, since unaligned accesses cause faults in that case.
>>
>> In summary, it would be nice if we could preserve the 'is ram"
>> annotation for regions that are not covered by the linear mapping.
>>
>> > While there aren't many users of page_is_ram() right now, I can see
>> > how in the future if new users are added they'd be extremely confused
>> > to find that page_is_ram(pfn) returns true but 'pfn' isn't accessible
>> > by the kernel proper.
>> >
>>
>> Well, who knows. page_is_ram() is poorly documented, and so is the
>> 'System RAM' iomem annotation that its default implementation relies
>> on.
>
> Sorry if this is a bit of a derailment, but perhaps now is a good
> opportunity to introduce something like:
>
> #ifndef page_is_linear_mapped
> #define page_is_linear_mapped page_is_ram
> #endif
>
> With documentation as to the semantic difference, and a conversion of
> existing users.
>

As I replied in the other thread, this does not cover all cases on
highmem platforms.