pipe: add documentation and comments

		page_cache_release(page);

}

/**

 * generic_pipe_buf_map - virtually map a pipe buffer

 * @pipe:	the pipe that the buffer belongs to

 * @buf:	the buffer that should be mapped

 * @atomic:	whether to use an atomic map

 *

 * Description:

 *	This function returns a kernel virtual address mapping for the

 *	passed in @pipe_buffer. If @atomic is set, an atomic map is provided

 *	and the caller has to be careful not to fault before calling

 *	the unmap function.

 *

 *	Note that this function occupies KM_USER0 if @atomic != 0.

 */

void *generic_pipe_buf_map(struct pipe_inode_info *pipe,

			   struct pipe_buffer *buf, int atomic)

{

	return kmap(buf->page);

}

/**

 * generic_pipe_buf_unmap - unmap a previously mapped pipe buffer

 * @pipe:	the pipe that the buffer belongs to

 * @buf:	the buffer that should be unmapped

 * @map_data:	the data that the mapping function returned

 *

 * Description:

 *	This function undoes the mapping that ->map() provided.

 */

void generic_pipe_buf_unmap(struct pipe_inode_info *pipe,

			    struct pipe_buffer *buf, void *map_data)

{

		kunmap(buf->page);

}

/**

 * generic_pipe_buf_steal - attempt to take ownership of a @pipe_buffer

 * @pipe:	the pipe that the buffer belongs to

 * @buf:	the buffer to attempt to steal

 *

 * Description:

 *	This function attempts to steal the @struct page attached to

 *	@buf. If successful, this function returns 0 and returns with

 *	the page locked. The caller may then reuse the page for whatever

 *	he wishes, the typical use is insertion into a different file

 *	page cache.

 */

int generic_pipe_buf_steal(struct pipe_inode_info *pipe,

			   struct pipe_buffer *buf)

{

	struct page *page = buf->page;

	/*

	 * A reference of one is golden, that means that the owner of this

	 * page is the only one holding a reference to it. lock the page

	 * and return OK.

	 */

	if (page_count(page) == 1) {

		lock_page(page);

		return 0;

	return 1;

}

void generic_pipe_buf_get(struct pipe_inode_info *info, struct pipe_buffer *buf)

/**

 * generic_pipe_buf_get - get a reference to a @struct pipe_buffer

 * @pipe:	the pipe that the buffer belongs to

 * @buf:	the buffer to get a reference to

 *

 * Description:

 *	This function grabs an extra reference to @buf. It's used in

 *	in the tee() system call, when we duplicate the buffers in one

 *	pipe into another.

 */

void generic_pipe_buf_get(struct pipe_inode_info *pipe, struct pipe_buffer *buf)

{

	page_cache_get(buf->page);

}

/**

 * generic_pipe_buf_confirm - verify contents of the pipe buffer

 * @pipe:	the pipe that the buffer belongs to

 * @buf:	the buffer to confirm

 *

 * Description:

 *	This function does nothing, because the generic pipe code uses

 *	pages that are always good when inserted into the pipe.

 */

int generic_pipe_buf_confirm(struct pipe_inode_info *info,

			     struct pipe_buffer *buf)

{

	buf->flags &= ~PIPE_BUF_FLAG_LRU;

}

/*

 * Check whether the contents of buf is OK to access. Since the content

 * is a page cache page, IO may be in flight.

 */

static int page_cache_pipe_buf_confirm(struct pipe_inode_info *pipe,

				       struct pipe_buffer *buf)

{

#define PIPE_BUF_FLAG_ATOMIC	0x02	/* was atomically mapped */

#define PIPE_BUF_FLAG_GIFT	0x04	/* page is a gift */

/**

 *	struct pipe_buffer - a linux kernel pipe buffer

 *	@page: the page containing the data for the pipe buffer

 *	@offset: offset of data inside the @page

 *	@len: length of data inside the @page

 *	@ops: operations associated with this buffer. See @pipe_buf_operations.

 *	@flags: pipe buffer flags. See above.

 *	@private: private data owned by the ops.

 **/

struct pipe_buffer {

	struct page *page;

	unsigned int offset, len;

	unsigned long private;

};

/**

 *	struct pipe_inode_info - a linux kernel pipe

 *	@wait: reader/writer wait point in case of empty/full pipe

 *	@nrbufs: the number of non-empty pipe buffers in this pipe

 *	@curbuf: the current pipe buffer entry

 *	@tmp_page: cached released page

 *	@readers: number of current readers of this pipe

 *	@writers: number of current writers of this pipe

 *	@waiting_writers: number of writers blocked waiting for room

 *	@r_counter: reader counter

 *	@w_counter: writer counter

 *	@fasync_readers: reader side fasync

 *	@fasync_writers: writer side fasync

 *	@inode: inode this pipe is attached to

 *	@bufs: the circular array of pipe buffers

 **/

struct pipe_inode_info {

	wait_queue_head_t wait;

	unsigned int nrbufs, curbuf;

 *	->unmap()

 *

 * That is, ->map() must be called on a confirmed buffer,

 * same goes for ->steal().

 * same goes for ->steal(). See below for the meaning of each

 * operation. Also see kerneldoc in fs/pipe.c for the pipe

 * and generic variants of these hooks.

 */

struct pipe_buf_operations {

	/*

	 * This is set to 1, if the generic pipe read/write may coalesce

	 * data into an existing buffer. If this is set to 0, a new pipe

	 * page segment is always used for new data.

	 */

	int can_merge;

	/*

	 * ->map() returns a virtual address mapping of the pipe buffer.

	 * The last integer flag reflects whether this should be an atomic

	 * mapping or not. The atomic map is faster, however you can't take

	 * page faults before calling ->unmap() again. So if you need to eg

	 * access user data through copy_to/from_user(), then you must get

	 * a non-atomic map. ->map() uses the KM_USER0 atomic slot for

	 * atomic maps, so you can't map more than one pipe_buffer at once

	 * and you have to be careful if mapping another page as source

	 * or destination for a copy (IOW, it has to use something else

	 * than KM_USER0).

	 */

	void * (*map)(struct pipe_inode_info *, struct pipe_buffer *, int);

	/*

	 * Undoes ->map(), finishes the virtual mapping of the pipe buffer.

	 */

	void (*unmap)(struct pipe_inode_info *, struct pipe_buffer *, void *);

	/*

	 * ->confirm() verifies that the data in the pipe buffer is there

	 * and that the contents are good. If the pages in the pipe belong

	 * to a file system, we may need to wait for IO completion in this

	 * hook. Returns 0 for good, or a negative error value in case of

	 * error.

	 */

	int (*confirm)(struct pipe_inode_info *, struct pipe_buffer *);

	/*

	 * When the contents of this pipe buffer has been completely

	 * consumed by a reader, ->release() is called.

	 */

	void (*release)(struct pipe_inode_info *, struct pipe_buffer *);

	/*

	 * Attempt to take ownership of the pipe buffer and its contents.

	 * ->steal() returns 0 for success, in which case the contents

	 * of the pipe (the buf->page) is locked and now completely owned

	 * by the caller. The page may then be transferred to a different

	 * mapping, the most often used case is insertion into different

	 * file address space cache.

	 */

	int (*steal)(struct pipe_inode_info *, struct pipe_buffer *);

	/*

	 * Get a reference to the pipe buffer.

	 */

	void (*get)(struct pipe_inode_info *, struct pipe_buffer *);

};