写文件的流程

很多文件系统都是通过generic_file_write()函数来实现文件对象的write方法，即write(库函数)->sys_write()->generic_file_write():

ssize_t generic_file_write(struct file *file, const char __user *buf,
			   size_t count, loff_t *ppos)
{
	struct address_space *mapping = file->f_mapping;
	struct inode *inode = mapping->host;
	ssize_t	ret;
	struct iovec local_iov = { .iov_base = (void __user *)buf,
					.iov_len = count };

	down(&inode->i_sem);
	ret = __generic_file_write_nolock(file, &local_iov, 1, ppos);
	up(&inode->i_sem);

	if (ret > 0 && ((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
		ssize_t err;

		err = sync_page_range(inode, mapping, *ppos - ret, ret);
		if (err < 0)
			ret = err;
	}
	return ret;
}

generic_file_write会调用__generic_file_write_nolock(),即write(库函数)->sys_write()->generic_file_write()->__generic_file_write_nolock:

ssize_t
__generic_file_write_nolock(struct file *file, const struct iovec *iov,
				unsigned long nr_segs, loff_t *ppos)
{
	struct kiocb kiocb;
	ssize_t ret;

	init_sync_kiocb(&kiocb, file);
	ret = __generic_file_aio_write_nolock(&kiocb, iov, nr_segs, ppos);
	if (ret == -EIOCBQUEUED)
		ret = wait_on_sync_kiocb(&kiocb);
	return ret;
}

write(库函数)->sys_write()->generic_file_write()->__generic_file_write_nolock()->__generic_file_aio_write_nolock():

ssize_t
__generic_file_aio_write_nolock(struct kiocb *iocb, const struct iovec *iov,
				unsigned long nr_segs, loff_t *ppos)
{
	struct file *file = iocb->ki_filp;
	struct address_space * mapping = file->f_mapping;
	size_t ocount;		/* original count */
	size_t count;		/* after file limit checks */
	struct inode 	*inode = mapping->host;
	unsigned long	seg;
	loff_t		pos;
	ssize_t		written;
	ssize_t		err;

	ocount = 0;
	for (seg = 0; seg < nr_segs; seg++) {
		const struct iovec *iv = &iov[seg];

		/*
		 * If any segment has a negative length, or the cumulative
		 * length ever wraps negative then return -EINVAL.
		 */
		ocount += iv->iov_len;
		if (unlikely((ssize_t)(ocount|iv->iov_len) < 0))
			return -EINVAL;
		if (access_ok(VERIFY_READ, iv->iov_base, iv->iov_len))
			continue;
		if (seg == 0)
			return -EFAULT;
		nr_segs = seg;
		ocount -= iv->iov_len;	/* This segment is no good */
		break;
	}

	count = ocount;
	pos = *ppos;

	/* We can write back this queue in page reclaim */
	current->backing_dev_info = mapping->backing_dev_info;
	written = 0;

	err = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
	if (err)
		goto out;

	if (count == 0)
		goto out;

	err = remove_suid(file->f_dentry);
	if (err)
		goto out;

	inode_update_time(inode, 1);

	/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
	if (unlikely(file->f_flags & O_DIRECT)) {
		written = generic_file_direct_write(iocb, iov,
				&nr_segs, pos, ppos, count, ocount);
		if (written < 0 || written == count)
			goto out;
		/*
		 * direct-io write to a hole: fall through to buffered I/O
		 * for completing the rest of the request.
		 */
		pos += written;
		count -= written;
	}

	written = generic_file_buffered_write(iocb, iov, nr_segs,
			pos, ppos, count, written);
out:
	current->backing_dev_info = NULL;
	return written ? written : err;
}

write(库函数)->sys_write()->generic_file_write()->__generic_file_write_nolock()->__generic_file_aio_write_nolock()->generic_file_buffered_write():

ssize_t
generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
		unsigned long nr_segs, loff_t pos, loff_t *ppos,
		size_t count, ssize_t written)
{
	struct file *file = iocb->ki_filp;
	struct address_space * mapping = file->f_mapping;
	struct address_space_operations *a_ops = mapping->a_ops;
	struct inode 	*inode = mapping->host;
	long		status = 0;
	struct page	*page;
	struct page	*cached_page = NULL;
	size_t		bytes;
	struct pagevec	lru_pvec;
	const struct iovec *cur_iov = iov; /* current iovec */
	size_t		iov_base = 0;	   /* offset in the current iovec */
	char __user	*buf;

	pagevec_init(&lru_pvec, 0);

	buf = iov->iov_base + written;	/* handle partial DIO write */
	do {
		unsigned long index;
		unsigned long offset;
		size_t copied;

		offset = (pos & (PAGE_CACHE_SIZE -1)); /* Within page */
		//获取要写的缓冲页面索引（怎样依据页索引在radix树中获取到指定页描写叙述符，ULK-PAGE600)
		index = pos >> PAGE_CACHE_SHIFT;
		bytes = PAGE_CACHE_SIZE - offset;
		//最后剩一点写入内容的处理
		if (bytes > count)
			bytes = count;

		/*
		 * Bring in the user page that we will copy from _first_.
		 * Otherwise there's a nasty deadlock on copying from the
		 * same page as we're writing to, without it being marked
		 * up-to-date.
		 */
		fault_in_pages_readable(buf, bytes);
		//在radix树里面查找要被写的page，假设不存在则创建一个，见以下分析
		page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);
		if (!page) {
			status = -ENOMEM;
			break;
		}
		//为这个page准备一组buffer_head结构，用于描写叙述组成这个page的数据块,见以下分析
		status = a_ops->prepare_write(file, page, offset, offset+bytes);
		if (unlikely(status)) {
			loff_t isize = i_size_read(inode);
			/*
			 * prepare_write() may have instantiated a few blocks
			 * outside i_size.  Trim these off again.
			 */
			unlock_page(page);
			page_cache_release(page);
			if (pos + bytes > isize)
				vmtruncate(inode, isize);
			break;
		}
		if (likely(nr_segs == 1))
			copied =  (page, offset,
							buf, bytes);
		else
			copied = filemap_copy_from_user_iovec(page, offset,
						cur_iov, iov_base, bytes);
		flush_dcache_page(page);
		//把基础缓冲区标记为脏，以便随后把他们都写到磁盘。
		status = a_ops->commit_write(file, page, offset, offset+bytes);
		if (likely(copied > 0)) {
			if (!status)
				status = copied;

			if (status >= 0) {
				written += status;
				count -= status;
				pos += status;
				buf += status;
				if (unlikely(nr_segs > 1))
					filemap_set_next_iovec(&cur_iov,
							&iov_base, status);
			}
		}
		if (unlikely(copied != bytes))
			if (status >= 0)
				status = -EFAULT;
		unlock_page(page);
		mark_page_accessed(page);
		page_cache_release(page);
		if (status < 0)
			break;
		balance_dirty_pages_ratelimited(mapping);
		cond_resched();
	} while (count);
	*ppos = pos;

	if (cached_page)
		page_cache_release(cached_page);

	/*
	 * For now, when the user asks for O_SYNC, we'll actually give O_DSYNC
	 */
	if (likely(status >= 0)) {
		if (unlikely((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
			if (!a_ops->writepage || !is_sync_kiocb(iocb))
				status = generic_osync_inode(inode, mapping,
						OSYNC_METADATA|OSYNC_DATA);
		}
  	}
	
	/*
	 * If we get here for O_DIRECT writes then we must have fallen through
	 * to buffered writes (block instantiation inside i_size).  So we sync
	 * the file data here, to try to honour O_DIRECT expectations.
	 */
	if (unlikely(file->f_flags & O_DIRECT) && written)
		status = filemap_write_and_wait(mapping);

	pagevec_lru_add(&lru_pvec);
	return written ? written : status;
}

static inline struct page *
__grab_cache_page(struct address_space *mapping, unsigned long index,
			struct page **cached_page, struct pagevec *lru_pvec)
{
	int err;
	struct page *page;
repeat:
	//依据address_space地址和缓冲页的索引，获取缓冲页面的描写叙述符(ULK-PAGE602)
	page = find_lock_page(mapping, index);
	if (!page) {
		if (!*cached_page) {
			*cached_page = page_cache_alloc(mapping);
			if (!*cached_page)
				return NULL;
		}
		//把一个新页的描写叙述符插入到页快速缓存--在radix树中出入新节点
		err = add_to_page_cache(*cached_page, mapping,
					index, GFP_KERNEL);
		if (err == -EEXIST)
			goto repeat;
		if (err == 0) {
			page = *cached_page;
			page_cache_get(page);
			if (!pagevec_add(lru_pvec, page))
				__pagevec_lru_add(lru_pvec);
			*cached_page = NULL;
		}
	}
	return page;
}

//prepare_write分析
address_space对象的prepare_write和commit_write方法专用于由generic_file_write()实现的通用写操作，这个函数适用于普通文件和块设备文件。每一个磁盘文件系统都定义了自己的prepare_write方法。与读操作类似，这种方法仅仅是普通函数的封装。比如，Ext2文件系统通过下列函数实现prepare_write方法
//在fs/buffer.c文件夹中
int ext2_prepare_write(struct file *file,struct page *page,unsigned from ,unsigned to)
{
return block_prepare_write(page,from,to,ext2_get_block);
}
一旦prepare_write,generic_file_write()函数就用存放在用户地址空间中的数据更新快速缓存页面。接下来，调用address_space对象的commit_write方法。这种方法由generic_commit_write()函数实现。generic_commit_write()函数运行例如以下步骤：
1.调用__block_commit_write()函数，运行例如以下步骤：
A.考虑页中受写操作影响的全部缓冲区；对于当中的每一个缓冲区，将相应缓冲区首部的BH_Uptodate和BH_Dirty标志置位。
B.标记对应索引节点为脏，将索引节点增加超级块脏的索引节点连接
C.假设缓冲区页中的全部缓冲区是最新的，则将PG_uptodate标志置位
D.将页的PG_Dirty标志置位，并在基树中将页标记成脏