页面的反向映射

反向映射是指根据struct page数据结构找到所有映射到这个page的vma，反向映射主要用于kswaped和页面迁移.
反向映射主要调用try_to_unmap来进行
其源码分析如下：

bool try_to_unmap(struct page *page, enum ttu_flags flags)
{
	struct rmap_walk_control rwc = {
		.rmap_one = try_to_unmap_one,
		.arg = (void *)flags,
		.done = page_mapcount_is_zero,
		.anon_lock = page_lock_anon_vma_read,
	};

	/*
	 * During exec, a temporary VMA is setup and later moved.
	 * The VMA is moved under the anon_vma lock but not the
	 * page tables leading to a race where migration cannot
	 * find the migration ptes. Rather than increasing the
	 * locking requirements of exec(), migration skips
	 * temporary VMAs until after exec() completes.
	 */
	if ((flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))
	    && !PageKsm(page) && PageAnon(page))
		rwc.invalid_vma = invalid_migration_vma;

	if (flags & TTU_RMAP_LOCKED)
		rmap_walk_locked(page, &rwc);
	else
		rmap_walk(page, &rwc);

	return !page_mapcount(page) ? true : false;
}
从上面可以看到如果page中包含TTU_RMAP_LOCKED则调用rmap_walk_locked 来断开映射到这个页面的vma，否则调用rmap_walk
这里以rmap_walk为例
void rmap_walk(struct page *page, struct rmap_walk_control *rwc)
{
	if (unlikely(PageKsm(page)))
		rmap_walk_ksm(page, rwc);
	else if (PageAnon(page))
		rmap_walk_anon(page, rwc, false);
	else
		rmap_walk_file(page, rwc, false);
}
从这个函数可以看到有二种场景会映射到page，分别是anon/file映射页面，通知不支持ksm映射页面

这里以rmap_walk_anon为例
static void rmap_walk_anon(struct page *page, struct rmap_walk_control *rwc,
		bool locked)
{
	struct anon_vma *anon_vma;
	pgoff_t pgoff_start, pgoff_end;
	struct anon_vma_chain *avc;
#根据page找到对应的anon_vma，这里有分为是否加锁两种情况
	if (locked) {
		anon_vma = page_anon_vma(page);
		/* anon_vma disappear under us? */
		VM_BUG_ON_PAGE(!anon_vma, page);
	} else {
		anon_vma = rmap_walk_anon_lock(page, rwc);
	}
	#如果没有找到对应的vma则退出
	if (!anon_vma)
		return;
	#得到这个page的起始和结束的index，这里通过hpage_nr_pages考虑了hugepage的case
	pgoff_start = page_to_pgoff(page);
	pgoff_end = pgoff_start + hpage_nr_pages(page) - 1;
	#遍历anon_vma->rb_root
	anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root,
			pgoff_start, pgoff_end) {
		#找到这段地址对应的vma
		struct vm_area_struct *vma = avc->vma;
		unsigned long address = vma_address(page, vma);

		cond_resched();

		if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
			continue;
		#调用try_to_unmap 中定义的rmap_one来断开vma和这个page的映射
		if (!rwc->rmap_one(page, vma, address, rwc->arg))
			break;
		if (rwc->done && rwc->done(page))
			break;
	}

	if (!locked)
		anon_vma_unlock_read(anon_vma);
}