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

[Ard Biesheuvel]

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.

> Wouldn't it make more sense to teach acpi_os_ioremap() about these
> special reserved regions outside of page_is_ram()?

Perhaps. But it would introduce EFI dependencies into that code.

The bottom line is that I would like to be able to remove UEFI
occupied or reserved regions from the linear mapping without breaking
ACPI, whose use of page_is_ram() results in alignment faults when
accessing such regions.

-- 
Ard.