/*
            
 * Allocate the accumulated non-linear sections, allocate a mem_map
 * for each and record the physical to section mapping.
 
            */
            void __init sparse_init(void)
{
        unsigned long pnum_end, pnum_begin, map_count = 1;
        int nid_begin;

        memblocks_present();    //【1】
        pnum_begin = first_present_section_nr();
        nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));

        /* Setup pageblock_order for HUGETLB_PAGE_SIZE_VARIABLE */
        set_pageblock_order();

        for_each_present_section_nr(pnum_begin + 1, pnum_end) {
                int nid = sparse_early_nid(__nr_to_section(pnum_end));

                if (nid == nid_begin) {
                        map_count++;
                        continue;
                }
                /* Init node with sections in range [pnum_begin, pnum_end) */
                sparse_init_nid(nid_begin, pnum_begin, pnum_end, map_count);   //【2】
                nid_begin = nid;
                pnum_begin = pnum_end;
                map_count = 1;
        }
        /* cover the last node */
        sparse_init_nid(nid_begin, pnum_begin, pnum_end, map_count);
        vmemmap_populate_print_last();
}

            /*
            
 * Initialize sparse on a specific node. The node spans [pnum_begin, pnum_end)
 * And number of present sections in this node is map_count.
 
            */
            static
            void __init sparse_init_nid(int nid, unsigned long pnum_begin,
                                   unsigned long pnum_end,
                                   unsigned long map_count)
{
        struct mem_section_usage *usage;
        unsigned long pnum;
        struct page *map;

        usage = sparse_early_usemaps_alloc_pgdat_section(NODE_DATA(nid),
                        mem_section_usage_size() * map_count); //【2.1】if (!usage) {
                pr_err("%s: node[%d] usemap allocation failed", __func__, nid);
                goto failed;
        }
        sparse_buffer_init(map_count * section_map_size(), nid); //【2.2】        for_each_present_section_nr(pnum_begin, pnum) {
                unsigned long pfn = section_nr_to_pfn(pnum);

                if (pnum >= pnum_end)
                        break;

                map = __populate_section_memmap(pfn, PAGES_PER_SECTION,    //【2.3】                                nid, NULL);
                if (!map) {
                        pr_err("%s: node[%d] memory map backing failed. Some memory will not be available.",
                               __func__, nid);
                        pnum_begin = pnum;
                        goto failed;
                }
                check_usemap_section_nr(nid, usage);
                sparse_init_one_section(__nr_to_section(pnum), pnum, map, usage,
                                SECTION_IS_EARLY);  //【2.4】
                usage = (void *) usage + mem_section_usage_size();
        }
        sparse_buffer_fini();     //【2.5】return;
failed:
        /* We failed to allocate, mark all the following pnums as not present */
        for_each_present_section_nr(pnum_begin, pnum) {
                struct mem_section *ms;

                if (pnum >= pnum_end)
                        break;
                ms = __nr_to_section(pnum);
                ms->section_mem_map = 0;
        }
}