Commit 0845718d authored by Jens Axboe's avatar Jens Axboe

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: default avatarJens Axboe <jens.axboe@oracle.com>
parent cac36bb0
......@@ -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)
{
......
......@@ -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)
{
......
......@@ -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 *);
};
......
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