Date:   Wed, 19 Jun 2019 10:13:50 +0800
From:   Wei Yang <richardw.yang@...ux.intel.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        Michal Hocko <mhocko@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v9 01/12] mm/sparsemem: Introduce struct mem_section_usage

On Tue, Jun 18, 2019 at 02:56:09PM -0700, Dan Williams wrote:
>On Sun, Jun 16, 2019 at 6:11 AM Wei Yang <richard.weiyang@...il.com> wrote:
>>
>> On Wed, Jun 05, 2019 at 02:57:54PM -0700, 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
>> >+
>> > #endif /* CONFIG_SPARSEMEM */
>> >
>> > #ifdef CONFIG_MEMORY_HOTPLUG
>> >diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> >index 427b79c39b3c..ac163f2f274f 100644
>> >--- a/include/linux/mmzone.h
>> >+++ b/include/linux/mmzone.h
>> >@@ -1161,6 +1161,44 @@ 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)))
>>
>> One pair of brackets could be removed, IMHO.
>
>Sure.
>
>>
>> >+
>> >+#if SUBSECTION_SHIFT > SECTION_SIZE_BITS
>> >+#error Subsection size exceeds section size
>> >+#else
>> >+#define SUBSECTIONS_PER_SECTION (1UL << (SECTION_SIZE_BITS - SUBSECTION_SHIFT))
>> >+#endif
>> >+
>> >+struct mem_section_usage {
>> >+      DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
>> >+      /* See declaration of similar field in struct zone */
>> >+      unsigned long pageblock_flags[0];
>> >+};
>> >+
>> > struct page;
>> > struct page_ext;
>> > struct mem_section {
>> >@@ -1178,8 +1216,7 @@ struct mem_section {
>> >        */
>> >       unsigned long section_mem_map;
>> >
>> >-      /* See declaration of similar field in struct zone */
>> >-      unsigned long *pageblock_flags;
>> >+      struct mem_section_usage *usage;
>> > #ifdef CONFIG_PAGE_EXTENSION
>> >       /*
>> >        * If SPARSEMEM, pgdat doesn't have page_ext pointer. We use
>> >@@ -1210,6 +1247,11 @@ extern struct mem_section **mem_section;
>> > extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
>> > #endif
>> >
>> >+static inline unsigned long *section_to_usemap(struct mem_section *ms)
>> >+{
>> >+      return ms->usage->pageblock_flags;
>>
>> Do we need to consider the case when ms->usage is NULL?
>
>No, this routine safely assumes it is always set.

Then everything looks good to me.

Reviewed-by: Wei Yang <richardw.yang@...ux.intel.com>

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-- 
Wei Yang
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