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>

diff --git a/fs/pipe.c b/fs/pipe.c
index 3694af10dd2c22423945a6ec1ef011a41563882e..d007830d9c870fabb4b887d3f81afcb8e7160642 100644 (file)
--- 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 c804121601b062dfec62b1cc2202e8e9243ce9a7..ed2ce995475c3f3cf6991de68cab9b38285ddc5d 100644 (file)
--- 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 cc09fe89bf0740795e65b9b33499378e95013877..8e4120285f72ef3379be0de768880b67453437c9 100644 (file)
--- 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 *);
 };