linux-kernel - Re: [PATCH v9 01/12] mm/sparsemem: Introduce struct mem_section

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Message-ID: <CAPcyv4hw2W3=CkrUmWtvu3cAdo3GLRhG0=G_RO7xQBugNB2htA@mail.gmail.com>
Date:   Tue, 18 Jun 2019 14:56:09 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Logan Gunthorpe <logang@...tatee.com>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 01/12] mm/sparsemem: Introduce struct mem_section_usage

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.