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

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

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.
>
> Based on an initial patch by Aneesh.

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

>
> Link: http://lore.kernel.org/r/CAPcyv4gBGNP95APYaBcsocEa50tQj9b5h__83vgngjq3ouGX_Q@mail.gmail.com
> Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Reported-by: Jeff Moyer <jmoyer@redhat.com>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  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(-)
>
> diff --git a/arch/powerpc/include/asm/io.h b/arch/powerpc/include/asm/io.h
> index a63ec938636d..0fa2dc483008 100644
> --- a/arch/powerpc/include/asm/io.h
> +++ b/arch/powerpc/include/asm/io.h
> @@ -734,6 +734,16 @@ extern void __iomem * __ioremap_at(phys_addr_t pa, void *ea,
>  				   unsigned long size, pgprot_t prot);
>  extern void __iounmap_at(void *ea, unsigned long size);
>  
> +#ifdef CONFIG_SPARSEMEM
> +static inline unsigned long memremap_compat_align(void)
> +{
> +	if (radix_enabled())
> +		return SUBSECTION_SIZE;
> +	return (1UL << mmu_psize_defs[mmu_linear_psize].shift);
> +}
> +#define memremap_compat_align memremap_compat_align
> +#endif
> +
>  /*
>   * When CONFIG_PPC_INDIRECT_PIO is set, we use the generic iomap implementation
>   * which needs some additional definitions here. They basically allow PIO
> diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c
> index b94f7a7e94b8..a5c25cb87116 100644
> --- a/drivers/nvdimm/pfn_devs.c
> +++ b/drivers/nvdimm/pfn_devs.c
> @@ -750,7 +750,7 @@ static int nd_pfn_init(struct nd_pfn *nd_pfn)
>  	start = nsio->res.start;
>  	size = resource_size(&nsio->res);
>  	npfns = PHYS_PFN(size - SZ_8K);
> -	align = max(nd_pfn->align, (1UL << SUBSECTION_SHIFT));
> +	align = max(nd_pfn->align, SUBSECTION_SIZE);
>  	end_trunc = start + size - ALIGN_DOWN(start + size, align);
>  	if (nd_pfn->mode == PFN_MODE_PMEM) {
>  		/*
> diff --git a/include/linux/io.h b/include/linux/io.h
> index 35e8d84935e0..ccd34519fad3 100644
> --- a/include/linux/io.h
> +++ b/include/linux/io.h
> @@ -6,6 +6,7 @@
>  #ifndef _LINUX_IO_H
>  #define _LINUX_IO_H
>  
> +#include <linux/mmzone.h>
>  #include <linux/types.h>
>  #include <linux/init.h>
>  #include <linux/bug.h>
> @@ -79,6 +80,28 @@ void *devm_memremap(struct device *dev, resource_size_t offset,
>  		size_t size, unsigned long flags);
>  void devm_memunmap(struct device *dev, void *addr);
>  
> +#ifndef memremap_compat_align
> +#ifdef CONFIG_SPARSEMEM
> +/*
> + * Minimum compatible alignment of the resource (start, end) across
> + * memremap interfaces (i.e. memremap + memremap_pages)
> + */
> +static inline unsigned long memremap_compat_align(void)
> +{
> +	return SUBSECTION_SIZE;
> +}
> +#else /* CONFIG_SPARSEMEM */
> +/*
> + * No ZONE_DEVICE / memremap_pages() support so the minimum mapping
> + * granularity is a single page.
> + */
> +static inline unsigned long memremap_compat_align(void)
> +{
> +	return PAGE_SIZE;
> +}
> +#endif /* CONFIG_SPARSEMEM */
> +#endif /* memremap_compat_align */
> +
>  #ifdef CONFIG_PCI
>  /*
>   * The PCI specifications (Rev 3.0, 3.2.5 "Transaction Ordering and
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 89d8ff06c9ce..b0de83620cd7 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1171,6 +1171,7 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
>  #define SECTION_ALIGN_DOWN(pfn)	((pfn) & PAGE_SECTION_MASK)
>  
>  #define SUBSECTION_SHIFT 21
> +#define SUBSECTION_SIZE (1UL << SUBSECTION_SHIFT)
>  
>  #define PFN_SUBSECTION_SHIFT (SUBSECTION_SHIFT - PAGE_SHIFT)
>  #define PAGES_PER_SUBSECTION (1UL << PFN_SUBSECTION_SHIFT)