pipe: add documentation and comments

As per Andrew Mortons request, here's a set of documentation for
the generic pipe_buf_operations hooks, the pipe, and pipe_buffer
structures.

Signed-off-by: Jens Axboe <[e-mail]>

fs/pipe.c

···
 		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)
 {

fs/splice.c

···
 	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)
 {

include/linux/pipe_fs_i.h

···
 #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 *);
 };