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>

2 files changed, 64 insertions, 1 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)
 {