[PATCH v2 9/9] mm: replace open coded page to virt conversion with page_to_virt()

[Ard Biesheuvel]

On 14 April 2016 at 17:25, Will Deacon <will.deacon at arm.com> wrote:
> On Wed, Mar 30, 2016 at 04:46:04PM +0200, Ard Biesheuvel wrote:
>> The open coded conversion from struct page address to virtual address in
>> lowmem_page_address() involves an intermediate conversion step to pfn
>> number/physical address. Since the placement of the struct page array
>> relative to the linear mapping may be completely independent from the
>> placement of physical RAM (as is that case for arm64 after commit
>> dfd55ad85e 'arm64: vmemmap: use virtual projection of linear region'),
>> the conversion to physical address and back again should factor out of
>> the equation, but unfortunately, the shifting and pointer arithmetic
>> involved prevent this from happening, and the resulting calculation
>> essentially subtracts the address of the start of physical memory and
>> adds it back again, in a way that prevents the compiler from optimizing
>> it away.
>>
>> Since the start of physical memory is not a build time constant on arm64,
>> the resulting conversion involves an unnecessary memory access, which
>> we would like to get rid of. So replace the open coded conversion with
>> a call to page_to_virt(), and use the open coded conversion as its
>> default definition, to be overriden by the architecture, if desired.
>> The existing arch specific definitions of page_to_virt are all equivalent
>> to this default definition, so by itself this patch is a no-op.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>
> Acked-by: Will Deacon <will.deacon at arm.com>
>
> I assume you'll post this patch (and the nios2/openrisc) patches as
> individual patches targetting the relevant trees?
>

Sure, as they are completely independent from the rest of the series.