linux-kernel - Re: [PATCH v3 1/5] mm/memremap_pages: Introduce memremap_compat

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Date:   Mon, 02 Mar 2020 17:31:58 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Dan Williams <dan.j.williams@...el.com>, linux-nvdimm@...ts.01.org
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 1/5] mm/memremap_pages: Introduce
 memremap_compat_align()

Dan Williams <dan.j.williams@...el.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.
>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>

> Link: http://lore.kernel.org/r/CAPcyv4gBGNP95APYaBcsocEa50tQj9b5h__83vgngjq3ouGX_Q@mail.gmail.com
> Reported-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> Reported-by: Jeff Moyer <jmoyer@...hat.com>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
>  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(-)
>