大页内存 初始化

hstates

所有的大页均放在 hstates 的全局变量中。
mm/hugetlb.c
struct hstate hstates[HUGE_MAX_HSTATE];

每个 hstates 数组的元素，均表示一种大小的page，其中 HUGE_MAX_HSTATE的值为2，这也就解释了为什么hugepage目前只支持2个pagesize。
当前系统有多少个类型 的hugepage，可以通过 /sys/kernel/mm/hugepages/查看，例如我当前系统只有20个大小为1G的Page：

$ls -l /sys/kernel/mm/hugepages/
total 0
drwxr-xr-x 2 root root 0 Nov  1 11:52 hugepages-1048576kB

$cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
20

初始化 hstates

系统启动后，会执行 parse_args函数解析一些启动参数
sudo grubby --update-kernel=ALL --args="hugepagesz=1G default_hugepagesz=1G hugepages=20"
我们设置的启动参数均会被解析，当解析 hugepagesz=时调用setup_hugepagesz函数，该函数 通过 hugetlb_add_hstate，初始化 hstates。

static __init int setup_hugepagesz(char *opt)
{
	unsigned long ps = memparse(opt, &opt);
	if (ps == PMD_SIZE) {
		hugetlb_add_hstate(PMD_SHIFT - PAGE_SHIFT);
	} else if (ps == PUD_SIZE && cpu_has_gbpages) {
		hugetlb_add_hstate(PUD_SHIFT - PAGE_SHIFT);
	} else {
		printk(KERN_ERR "hugepagesz: Unsupported page size %lu M\n",
			ps >> 20);
		return 0;
	}
	return 1;
}

可见，设置的size不是随意设置，其大小必须要么是PUD_SIZE或者PMD_SIZE。对于X64来说，就是1G或者2M。

申请大页内存

解析 到 hugepages=命令的时候，会调用 hugetlb_nrpages_setup

static void __init hugetlb_hstate_alloc_pages(struct hstate *h)
{
	unsigned long i;

    //hugepages=10，则申请 10个 大页
	for (i = 0; i < h->max_huge_pages; ++i) {
		if (hstate_is_gigantic(h)) {
			if (!alloc_bootmem_huge_page(h))
				break;
		} else if (!alloc_fresh_huge_page(h,
					 &node_states[N_MEMORY]))
			break;
	}
	h->max_huge_pages = i;
}

alloc_bootmem_huge_page函数申请一块大内存，然后放在全局队列 huge_boot_pages中。

组织大页

/* Put bootmem huge pages into the standard lists after mem_map is up */
static void __init gather_bootmem_prealloc(void)
{
	struct huge_bootmem_page *m;

	list_for_each_entry(m, &huge_boot_pages, list) {
		struct hstate *h = m->hstate;
		struct page *page;

#ifdef CONFIG_HIGHMEM
		page = pfn_to_page(m->phys >> PAGE_SHIFT);
		free_bootmem_late((unsigned long)m,
				  sizeof(struct huge_bootmem_page));
#else
		page = virt_to_page(m);
#endif
		WARN_ON(page_count(page) != 1);
		prep_compound_huge_page(page, h->order);
		WARN_ON(PageReserved(page));
		prep_new_huge_page(h, page, page_to_nid(page));
		/*
		 * If we had gigantic hugepages allocated at boot time, we need
		 * to restore the 'stolen' pages to totalram_pages in order to
		 * fix confusing memory reports from free(1) and another
		 * side-effects, like CommitLimit going negative.
		 */
		if (hstate_is_gigantic(h))
			totalram_pages += 1 << h->order;
	}
}

实际是在prep_new_huge_page函数中，设置page的析构函数为free_huge_page。将大页page，放入 h->hugepage_freelists