Re: [PATCH v2 1/4] mm/memremap_pages: Introduce memremap_compat_align()

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Feb 13, 2020 at 8:58 AM Jeff Moyer <jmoyer@redhat.com> wrote:
>
> Dan Williams <dan.j.williams@intel.com> 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.
>
> I have just a couple of questions.
>
> First, can you please add a comment above the generic implementation of
> memremap_compat_align describing its purpose, and why a platform might
> want to override it?

Sure, how about:

/*
 * The memremap() and memremap_pages() interfaces are alternately used
 * to map persistent memory namespaces. These interfaces place different
 * constraints on the alignment and size of the mapping (namespace).
 * memremap() can map individual PAGE_SIZE pages. memremap_pages() can
 * only map subsections (2MB), and at least one architecture (PowerPC)
 * the minimum mapping granularity of memremap_pages() is 16MB.
 *
 * The role of memremap_compat_align() is to communicate the minimum
 * arch supported alignment of a namespace such that it can freely
 * switch modes without violating the arch constraint. Namely, do not
 * allow a namespace to be PAGE_SIZE aligned since that namespace may be
 * reconfigured into a mode that requires SUBSECTION_SIZE alignment.
 */

> Second, I will take it at face value that the power architecture
> requires a 16MB alignment, but it's not clear to me why mmu_linear_psize
> was chosen to represent that.  What's the relationship, there, and can
> we please have a comment explaining it?

Aneesh, can you help here?
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