linux-kernel - Re: [PATCH v8 01/12] mm/sparsemem: Introduce struct mem_section

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Date:   Fri, 10 May 2019 15:30:29 +0200
From:   osalvador@...e.de
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Logan Gunthorpe <logang@...tatee.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-nvdimm@...ts.01.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, owner-linux-mm@...ck.org
Subject: Re: [PATCH v8 01/12] mm/sparsemem: Introduce struct mem_section_usage

On 2019-05-07 01:39, Dan Williams wrote:
> Towards enabling memory hotplug to track partial population of a
> section, introduce 'struct mem_section_usage'.
> 
> A pointer to a 'struct mem_section_usage' instance replaces the 
> existing
> pointer to a 'pageblock_flags' bitmap. Effectively it adds one more
> 'unsigned long' beyond the 'pageblock_flags' (usemap) allocation to
> house a new 'subsection_map' bitmap.  The new bitmap enables the memory
> hot{plug,remove} implementation to act on incremental sub-divisions of 
> a
> section.
> 
> The default SUBSECTION_SHIFT is chosen to keep the 'subsection_map' no
> larger than a single 'unsigned long' on the major architectures.
> Alternatively an architecture can define ARCH_SUBSECTION_SHIFT to
> override the default PMD_SHIFT. Note that PowerPC needs to use
> ARCH_SUBSECTION_SHIFT to workaround PMD_SHIFT being a non-constant
> expression on PowerPC.
> 
> The primary motivation for this functionality is to support platforms
> that mix "System RAM" and "Persistent Memory" within a single section,
> or multiple PMEM ranges with different mapping lifetimes within a 
> single
> section. The section restriction for hotplug has caused an ongoing saga
> of hacks and bugs for devm_memremap_pages() users.
> 
> Beyond the fixups to teach existing paths how to retrieve the 'usemap'
> from a section, and updates to usemap allocation path, there are no
> expected behavior changes.
> 
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Logan Gunthorpe <logang@...tatee.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
>  arch/powerpc/include/asm/sparsemem.h |    3 +
>  include/linux/mmzone.h               |   48 +++++++++++++++++++-
>  mm/memory_hotplug.c                  |   18 ++++----
>  mm/page_alloc.c                      |    2 -
>  mm/sparse.c                          |   81 
> +++++++++++++++++-----------------
>  5 files changed, 99 insertions(+), 53 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/sparsemem.h
> b/arch/powerpc/include/asm/sparsemem.h
> index 3192d454a733..1aa3c9303bf8 100644
> --- a/arch/powerpc/include/asm/sparsemem.h
> +++ b/arch/powerpc/include/asm/sparsemem.h
> @@ -10,6 +10,9 @@
>   */
>  #define SECTION_SIZE_BITS       24
> 
> +/* Reflect the largest possible PMD-size as the subsection-size 
> constant */
> +#define ARCH_SUBSECTION_SHIFT 24
> +

I guess this is done because PMD_SHIFT is defined at runtime rather at 
compile time,
right?


>  #endif /* CONFIG_SPARSEMEM */
> 
>  #ifdef CONFIG_MEMORY_HOTPLUG
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 70394cabaf4e..ef8d878079f9 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1160,6 +1160,44 @@ static inline unsigned long
> section_nr_to_pfn(unsigned long sec)
>  #define SECTION_ALIGN_UP(pfn)	(((pfn) + PAGES_PER_SECTION - 1) &
> PAGE_SECTION_MASK)
>  #define SECTION_ALIGN_DOWN(pfn)	((pfn) & PAGE_SECTION_MASK)
> 
> +/*
> + * SUBSECTION_SHIFT must be constant since it is used to declare
> + * subsection_map and related bitmaps without triggering the 
> generation
> + * of variable-length arrays. The most natural size for a subsection 
> is
> + * a PMD-page. For architectures that do not have a constant PMD-size
> + * ARCH_SUBSECTION_SHIFT can be set to a constant max size, or 
> otherwise
> + * fallback to 2MB.
> + */
> +#if defined(ARCH_SUBSECTION_SHIFT)
> +#define SUBSECTION_SHIFT (ARCH_SUBSECTION_SHIFT)
> +#elif defined(PMD_SHIFT)
> +#define SUBSECTION_SHIFT (PMD_SHIFT)
> +#else
> +/*
> + * Memory hotplug enabled platforms avoid this default because they
> + * either define ARCH_SUBSECTION_SHIFT, or PMD_SHIFT is a constant, 
> but
> + * this is kept as a backstop to allow compilation on
> + * !ARCH_ENABLE_MEMORY_HOTPLUG archs.
> + */
> +#define SUBSECTION_SHIFT 21
> +#endif
> +
> +#define PFN_SUBSECTION_SHIFT (SUBSECTION_SHIFT - PAGE_SHIFT)
> +#define PAGES_PER_SUBSECTION (1UL << PFN_SUBSECTION_SHIFT)
> +#define PAGE_SUBSECTION_MASK ((~(PAGES_PER_SUBSECTION-1)))
> +
> +#if SUBSECTION_SHIFT > SECTION_SIZE_BITS
> +#error Subsection size exceeds section size
> +#else
> +#define SUBSECTIONS_PER_SECTION (1UL << (SECTION_SIZE_BITS - 
> SUBSECTION_SHIFT))
> +#endif

On powerpc, SUBSECTIONS_PER_SECTION will equal 1 (so one big section), 
is that to be expected?
Will subsection_map_init handle this right?