Re: [PATCH 2/5] mm/memremap_pages: Introduce memremap_compat_align()

[Dan Williams]

On Tue, Feb 4, 2020 at 7:05 PM Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
>
> Dan Williams <dan.j.williams@xxxxxxxxx> writes:
> > The "sub-section memory hotplug" facility allows memremap_pages() users
> > like libnvdimm to compensate for hardware platforms like x86 that have a
> > section size larger than their hardware memory mapping granularity.  The
> > compensation that sub-section support affords is being tolerant of
> > physical memory resources shifting by units smaller (64MiB on x86) than
> > the memory-hotplug section size (128 MiB). Where the platform
> > physical-memory mapping granularity is limited by the number and
> > capability of address-decode-registers in the memory controller.
> >
> > While the sub-section support allows memremap_pages() to operate on
> > sub-section (2MiB) granularity, the Power architecture may still
> > require 16MiB alignment on "!radix_enabled()" platforms.
> >
> > In order for libnvdimm to be able to detect and manage this per-arch
> > limitation, introduce memremap_compat_align() as a common minimum
> > alignment across all driver-facing memory-mapping interfaces, and let
> > Power override it to 16MiB in the "!radix_enabled()" case.
> >
> > The assumption / requirement for 16MiB to be a viable
> > memremap_compat_align() value is that Power does not have platforms
> > where its equivalent of address-decode-registers never hardware remaps a
> > persistent memory resource on smaller than 16MiB boundaries.
> >
> > Based on an initial patch by Aneesh.
> >
> > Link: http://lore.kernel.org/r/CAPcyv4gBGNP95APYaBcsocEa50tQj9b5h__83vgngjq3ouGX_Q@xxxxxxxxxxxxxx
> > Reported-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> > Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
> > Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Paul Mackerras <paulus@xxxxxxxxx>
> > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > ---
> >  arch/powerpc/include/asm/io.h |   10 ++++++++++
> >  drivers/nvdimm/pfn_devs.c     |    2 +-
> >  include/linux/io.h            |   23 +++++++++++++++++++++++
> >  include/linux/mmzone.h        |    1 +
> >  4 files changed, 35 insertions(+), 1 deletion(-)
>
> The powerpc change here looks fine to me.
>
> Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> (powerpc)

Thanks Michael, unfortunately the kbuild robot just woke up and said
that mips does not like including mmzone.h from io.h. The
entanglements look intractable.

Is there a file I can stash a strong definition of
memremap_compat_align(), maybe arch/powerpc/mm/mem.c? Then I can put a
generic __weak definition in mm/memremap.c rather than play header
file include games.