Re: [PATCH v3 1/5] mm/memremap_pages: Introduce memremap_compat_align()

Dan Williams [off-list ref] 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. Note that I
tried my best to not add a new Kconfig symbol, but header include
entanglements defeated the #ifndef memremap_compat_align design pattern
and the need to export it defeats the __weak design pattern for arch
overrides.

Based on an initial patch by Aneesh.

Reviewed-by: Aneesh Kumar K.V <redacted>

Link: http://lore.kernel.org/r/CAPcyv4gBGNP95APYaBcsocEa50tQj9b5h__83vgngjq3ouGX_Q@mail.gmail.com (local)
Reported-by: Aneesh Kumar K.V <redacted>
Reported-by: Jeff Moyer <redacted>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Signed-off-by: Dan Williams <redacted>
---
 arch/powerpc/Kconfig      |    1 +
 arch/powerpc/mm/ioremap.c |   21 +++++++++++++++++++++
 drivers/nvdimm/pfn_devs.c |    2 +-
 include/linux/memremap.h  |    8 ++++++++
 include/linux/mmzone.h    |    1 +
 lib/Kconfig               |    3 +++
 mm/memremap.c             |   23 +++++++++++++++++++++++
 7 files changed, 58 insertions(+), 1 deletion(-)