alloc_node_mem_map的offset作用

看这个函数过程中看到pgdat->node_mem_map = map + offset;这个为什么加offset呢？

原来是计算本节点的mem_map数组的size的时候，start对齐到~(MAX_ORDER_NR_PAGES - 1)，无形中在开头和结尾增加了mem_map的大小，

而且pgdat->node_mem_map指向的mem_map属于本节点的第一个页面的描述符。因此需要加上offset。

offset的单位也很特别：

offset = pgdat->node_start_pfn - start; 刚好是页框单位；

static void __init_refok alloc_node_mem_map(struct pglist_data *pgdat)

{
unsigned long __maybe_unused start = 0;
unsigned long __maybe_unused offset = 0;


/* Skip empty nodes */
if (!pgdat->node_spanned_pages)
return;


#ifdef CONFIG_FLAT_NODE_MEM_MAP
start = pgdat->node_start_pfn & ~(MAX_ORDER_NR_PAGES - 1);
offset = pgdat->node_start_pfn - start;
/* ia64 gets its own node_mem_map, before this, without bootmem */
if (!pgdat->node_mem_map) {
unsigned long size, end;
struct page *map;


/*
* The zone's endpoints aren't required to be MAX_ORDER
* aligned but the node_mem_map endpoints must be in order
* for the buddy allocator to function correctly.
*/
end = pgdat_end_pfn(pgdat);
end = ALIGN(end, MAX_ORDER_NR_PAGES);
size =  (end - start) * sizeof(struct page);
map = alloc_remap(pgdat->node_id, size);
if (!map)
map = memblock_virt_alloc_node_nopanic(size,
      pgdat->node_id);
pgdat->node_mem_map = map + offset;
}
#ifndef CONFIG_NEED_MULTIPLE_NODES
/*
* With no DISCONTIG, the global mem_map is just set as node 0's
*/
if (pgdat == NODE_DATA(0)) {
mem_map = NODE_DATA(0)->node_mem_map;
#if defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP) || defined(CONFIG_FLATMEM)
if (page_to_pfn(mem_map) != pgdat->node_start_pfn)
mem_map -= offset;
#endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */
}
#endif
#endif /* CONFIG_FLAT_NODE_MEM_MAP */
}