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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);}

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