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从下面这段code 可以知道目前kernel支持3中内存模型分别是flatmem/discontigmem/spare ，其中spare有分为vmemmap和sparemem#if defined(CONFIG_FLATMEM)#define __pfn_to_page(pfn)	(mem_map + ((pfn) - ARCH_PFN_OFFSET))#define __page_to_pfn(page)	((unsigned long)((page) - mem_map) + \				 ARCH_PFN_OFFSET)#elif defined(CONFIG_DISCONTIGMEM)#define __pfn_to_page(pfn)			\({	unsigned long __pfn = (pfn);		\	unsigned long __nid = arch_pfn_to_nid(__pfn);  \	NODE_DATA(__nid)->node_mem_map + arch_local_page_offset(__pfn, __nid);\})#elif defined(CONFIG_SPARSEMEM_VMEMMAP)/* memmap is virtually contiguous.  */#define __pfn_to_page(pfn)	(vmemmap + (pfn))#define __page_to_pfn(page)	(unsigned long)((page) - vmemmap)#elif defined(CONFIG_SPARSEMEM)/* * Note: section's mem_map is encoded to reflect its start_pfn. * section[i].section_mem_map == mem_map's address - start_pfn; */#define __page_to_pfn(pg)					\({	const struct page *__pg = (pg);				\	int __sec = page_to_section(__pg);			\	(unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec)));	\})#endif /* CONFIG_FLATMEM/DISCONTIGMEM/SPARSEMEM */flatmem是指整个目录内存是连续的，整个系统只有一个内存节点discontigmem是侄儿物理内存是不连续的，物理内存中肯能有空洞，系统可以有多个节点，每个节点内部的内存是连续的，常用于numa和smpspare 是指内存是不连续，同时节点内的内存也是不连续的。从上面的看出内存模型主要是用于保存pfn和page的一个映射关系，这里有一个mem_map的数据结构永远保存这种关系这里以flat模型为例看看mem_map的初始化#ifdef CONFIG_FLAT_NODE_MEM_MAPstatic void __ref alloc_node_mem_map(struct pglist_data *pgdat){	unsigned long __maybe_unused start = 0;	unsigned long __maybe_unused offset = 0;	/* Skip empty nodes */	#如果这个节点没有内存则退出	if (!pgdat->node_spanned_pages)		return;	#得到这个节点内存的起始地址和offset	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 */	#如果这个节点还没有内存	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.		 */		 #计算这个节点需要保存的页的个数，算法就是结束地址减去起始地址乘以page 占用的size		end = pgdat_end_pfn(pgdat);		end = ALIGN(end, MAX_ORDER_NR_PAGES);		size =  (end - start) * sizeof(struct page);		#申请要保存这些页所要的内存		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;	}	pr_debug("%s: node %d, pgdat %08lx, node_mem_map %08lx\n",				__func__, pgdat->node_id, (unsigned long)pgdat,				(unsigned long)pgdat->node_mem_map);#ifndef CONFIG_NEED_MULTIPLE_NODES	/*	 * With no DISCONTIG, the global mem_map is just set as node 0's	 */	 #平坦模型的话，保存node 0的page	if (pgdat == NODE_DATA(0)) {	#将mem_map指向这个节点的map，至此完成赋值		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}#elsestatic void __ref alloc_node_mem_map(struct pglist_data *pgdat) { }#endif /* CONFIG_FLAT_NODE_MEM_MAP */

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