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 <jens.axboe@oracle.com>

3 files changed, 140 insertions, 2 deletions

diff --git a/fs/pipe.c b/fs/pipe.c
index 3694af10dd2c..d007830d9c87 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -164,6 +164,20 @@ static void anon_pipe_buf_release(struct pipe_inode_info *pipe,
                 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)
 {
@@ -175,6 +189,15 @@ void *generic_pipe_buf_map(struct pipe_inode_info *pipe,
         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)
 {
@@ -185,11 +208,28 @@ void generic_pipe_buf_unmap(struct pipe_inode_info *pipe,
                 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;
@@ -198,11 +238,30 @@ int generic_pipe_buf_steal(struct pipe_inode_info *pipe,
         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)
 {
diff --git a/fs/splice.c b/fs/splice.c
index c804121601b0..ed2ce995475c 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -85,6 +85,10 @@ static void page_cache_pipe_buf_release(struct pipe_inode_info *pipe,
         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)
 {
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index cc09fe89bf07..8e4120285f72 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -9,6 +9,15 @@
 #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;
@@ -17,6 +26,22 @@ struct pipe_buffer {
         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;
@@ -43,15 +68,65 @@ struct pipe_inode_info {
  *      ->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 *);
 };