内存管理API之get_user_pages函数

long get_user_pages(unsigned long start, unsigned long nr_pages,
		unsigned int gup_flags, struct page **pages,
		struct vm_area_struct **vmas)
用于将用户空间的页映射到内存，并返回它们页结构的指针，即get_user_pages的第四个形参struct page **pages
其源码分析如下：
long get_user_pages(unsigned long start, unsigned long nr_pages,
		unsigned int gup_flags, struct page **pages,
		struct vm_area_struct **vmas)
{
	#可见这个函数在带当前进程中映射内存
	return __get_user_pages_locked(current, current->mm, start, nr_pages,
				       pages, vmas, NULL, false,
				       gup_flags | FOLL_TOUCH);
}
static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
						struct mm_struct *mm,
						unsigned long start,
						unsigned long nr_pages,
						struct page **pages,
						struct vm_area_struct **vmas,
						int *locked, bool notify_drop,
						unsigned int flags)
{
	long ret, pages_done;
	bool lock_dropped;
	#本例中locked 为null
	if (locked) {
		/* if VM_FAULT_RETRY can be returned, vmas become invalid */
		BUG_ON(vmas);
		/* check caller initialized locked */
		BUG_ON(*locked != 1);
	}
 
	if (pages)
		flags |= FOLL_GET;
 
	pages_done = 0;
	lock_dropped = false;
	for (;;) {
		#从start 开始映射nr_pages的页，将页指针保存在pages中，这里的ret表示映射成功页的个数
		ret = __get_user_pages(tsk, mm, start, nr_pages, flags, pages,
				       vmas, locked);
		if (!locked)
			/* VM_FAULT_RETRY couldn't trigger, bypass */
			return ret;
 
		/* VM_FAULT_RETRY cannot return errors */
		if (!*locked) {
			BUG_ON(ret < 0);
			BUG_ON(ret >= nr_pages);
		}
 
		if (!pages)
			/* If it's a prefault don't insist harder */
			return ret;
		#这个if 条件成立，说明__get_user_pages 成功映射了ret个页
		if (ret > 0) {
			nr_pages -= ret;
			pages_done += ret;
			if (!nr_pages)
				break;
		}
		if (*locked) {
			/* VM_FAULT_RETRY didn't trigger */
			if (!pages_done)
				pages_done = ret;
			break;
		}
		/* VM_FAULT_RETRY triggered, so seek to the faulting offset */
		pages += ret;
		start += ret << PAGE_SHIFT;
 
		/*
		 * Repeat on the address that fired VM_FAULT_RETRY
		 * without FAULT_FLAG_ALLOW_RETRY but with
		 * FAULT_FLAG_TRIED.
		 */
		*locked = 1;
		lock_dropped = true;
		down_read(&mm->mmap_sem);
		#再从start 开始映射一个页，正常情况下这里会failed。
		ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
				       pages, NULL, NULL);
		if (ret != 1) {
			BUG_ON(ret > 1);
			if (!pages_done)
				pages_done = ret;
			break;
			#正常情况下for 死循环会从这里退出
		}
		#如果程序跑到这里，说明第一次映射nr_pages 页的时候failed了。但是第二次调用__get_user_pages 
		#映射单页确成功了。因此总的要映射的page 数减一，且start起始地址加一个page 后，重新开始映射.
		nr_pages--;
		pages_done++;
		if (!nr_pages)
			break;
		pages++;
		start += PAGE_SIZE;
	}
	if (notify_drop && lock_dropped && *locked) {
		/*
		 * We must let the caller know we temporarily dropped the lock
		 * and so the critical section protected by it was lost.
		 */
		up_read(&mm->mmap_sem);
		*locked = 0;
	}
	#这里的pages_done 表示映射成功的页的个数.
	return pages_done;
}           

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原文作者：首页 - 内核技术中文网 - 构建全国最权威的内核技术交流分享论坛

原文地址：内存管理API之get_user_pages - 圈点 - 内核技术中文网 - 构建全国最权威的内核技术交流分享论坛（版权归原文作者所有，侵权联系删除）