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 3694af1..d007830 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -164,6 +164,20 @@
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 @@
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 @@
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 @@
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 c8041216..ed2ce99 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -85,6 +85,10 @@
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 cc09fe8..8e41202 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 @@
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 @@
* ->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 *);
};