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    Subject[PATCH v8 03/12] mm/sparsemem: Add helpers track active portions of a section at boot
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    Prepare for hot{plug,remove} of sub-ranges of a section by tracking a
    sub-section active bitmask, each bit representing a PMD_SIZE span of the
    architecture's memory hotplug section size.

    The implications of a partially populated section is that pfn_valid()
    needs to go beyond a valid_section() check and read the sub-section
    active ranges from the bitmask. The expectation is that the bitmask
    (subsection_map) fits in the same cacheline as the valid_section() data,
    so the incremental performance overhead to pfn_valid() should be
    negligible.

    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Logan Gunthorpe <logang@deltatee.com>
    Cc: Oscar Salvador <osalvador@suse.de>
    Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
    Tested-by: Jane Chu <jane.chu@oracle.com>
    Signed-off-by: Dan Williams <dan.j.williams@intel.com>
    ---
    include/linux/mmzone.h | 29 ++++++++++++++++++++++++++++-
    mm/page_alloc.c | 4 +++-
    mm/sparse.c | 29 +++++++++++++++++++++++++++++
    3 files changed, 60 insertions(+), 2 deletions(-)

    diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
    index ac163f2f274f..6dd52d544857 100644
    --- a/include/linux/mmzone.h
    +++ b/include/linux/mmzone.h
    @@ -1199,6 +1199,8 @@ struct mem_section_usage {
    unsigned long pageblock_flags[0];
    };

    +void subsection_map_init(unsigned long pfn, unsigned long nr_pages);
    +
    struct page;
    struct page_ext;
    struct mem_section {
    @@ -1336,12 +1338,36 @@ static inline struct mem_section *__pfn_to_section(unsigned long pfn)

    extern int __highest_present_section_nr;

    +static inline int subsection_map_index(unsigned long pfn)
    +{
    + return (pfn & ~(PAGE_SECTION_MASK)) / PAGES_PER_SUBSECTION;
    +}
    +
    +#ifdef CONFIG_SPARSEMEM_VMEMMAP
    +static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
    +{
    + int idx = subsection_map_index(pfn);
    +
    + return test_bit(idx, ms->usage->subsection_map);
    +}
    +#else
    +static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
    +{
    + return 1;
    +}
    +#endif
    +
    #ifndef CONFIG_HAVE_ARCH_PFN_VALID
    static inline int pfn_valid(unsigned long pfn)
    {
    + struct mem_section *ms;
    +
    if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
    return 0;
    - return valid_section(__nr_to_section(pfn_to_section_nr(pfn)));
    + ms = __nr_to_section(pfn_to_section_nr(pfn));
    + if (!valid_section(ms))
    + return 0;
    + return pfn_section_valid(ms, pfn);
    }
    #endif

    @@ -1373,6 +1399,7 @@ void sparse_init(void);
    #define sparse_init() do {} while (0)
    #define sparse_index_init(_sec, _nid) do {} while (0)
    #define pfn_present pfn_valid
    +#define subsection_map_init(_pfn, _nr_pages) do {} while (0)
    #endif /* CONFIG_SPARSEMEM */

    /*
    diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    index 61c2b54a5b61..13816c5a51eb 100644
    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -7291,10 +7291,12 @@ void __init free_area_init_nodes(unsigned long *max_zone_pfn)

    /* Print out the early node map */
    pr_info("Early memory node ranges\n");
    - for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid)
    + for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid) {
    pr_info(" node %3d: [mem %#018Lx-%#018Lx]\n", nid,
    (u64)start_pfn << PAGE_SHIFT,
    ((u64)end_pfn << PAGE_SHIFT) - 1);
    + subsection_map_init(start_pfn, end_pfn - start_pfn);
    + }

    /* Initialise every node */
    mminit_verify_pageflags_layout();
    diff --git a/mm/sparse.c b/mm/sparse.c
    index f87de7ad32c8..ac47a48050c7 100644
    --- a/mm/sparse.c
    +++ b/mm/sparse.c
    @@ -210,6 +210,35 @@ static inline unsigned long first_present_section_nr(void)
    return next_present_section_nr(-1);
    }

    +void subsection_map_init(unsigned long pfn, unsigned long nr_pages)
    +{
    + int end_sec = pfn_to_section_nr(pfn + nr_pages - 1);
    + int i, start_sec = pfn_to_section_nr(pfn);
    +
    + if (!nr_pages)
    + return;
    +
    + for (i = start_sec; i <= end_sec; i++) {
    + int idx, end;
    + unsigned long pfns;
    + struct mem_section *ms;
    +
    + idx = subsection_map_index(pfn);
    + pfns = min(nr_pages, PAGES_PER_SECTION
    + - (pfn & ~PAGE_SECTION_MASK));
    + end = subsection_map_index(pfn + pfns - 1);
    +
    + ms = __nr_to_section(i);
    + bitmap_set(ms->usage->subsection_map, idx, end - idx + 1);
    +
    + pr_debug("%s: sec: %d pfns: %ld set(%d, %d)\n", __func__, i,
    + pfns, idx, end - idx + 1);
    +
    + pfn += pfns;
    + nr_pages -= pfns;
    + }
    +}
    +
    /* Record a memory area against a node. */
    void __init memory_present(int nid, unsigned long start, unsigned long end)
    {
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