[PATCH] splice: add comments documenting more of the code
Hopefully this will make Andrew a little more happy.
Signed-off-by: Jens Axboe <axboe@suse.de>
diff --git a/fs/splice.c b/fs/splice.c
index 07f4d86..3459192 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -37,6 +37,12 @@
loff_t pos; /* file position */
};
+/*
+ * Attempt to steal a page from a pipe buffer. This should perhaps go into
+ * a vm helper function, it's already simplified quite a bit by the
+ * addition of remove_mapping(). If success is returned, the caller may
+ * attempt to reuse this page for another destination.
+ */
static int page_cache_pipe_buf_steal(struct pipe_inode_info *info,
struct pipe_buffer *buf)
{
@@ -108,6 +114,10 @@
.steal = page_cache_pipe_buf_steal,
};
+/*
+ * Pipe output worker. This sets up our pipe format with the page cache
+ * pipe buffer operations. Otherwise very similar to the regular pipe_writev().
+ */
static ssize_t move_to_pipe(struct inode *inode, struct page **pages,
int nr_pages, unsigned long offset,
unsigned long len, unsigned int flags)
@@ -292,6 +302,16 @@
return move_to_pipe(pipe, pages, i, offset, len, flags);
}
+/**
+ * generic_file_splice_read - splice data from file to a pipe
+ * @in: file to splice from
+ * @pipe: pipe to splice to
+ * @len: number of bytes to splice
+ * @flags: splice modifier flags
+ *
+ * Will read pages from given file and fill them into a pipe.
+ *
+ */
ssize_t generic_file_splice_read(struct file *in, struct inode *pipe,
size_t len, unsigned int flags)
{
@@ -370,10 +390,12 @@
* - Destination page does not exist, we can add the pipe page to
* the page cache and avoid the copy.
*
- * For now we just do the slower thing and always copy pages over, it's
- * easier than migrating pages from the pipe to the target file. For the
- * case of doing file | file splicing, the migrate approach had some LRU
- * nastiness...
+ * If asked to move pages to the output file (SPLICE_F_MOVE is set in
+ * sd->flags), we attempt to migrate pages from the pipe to the output
+ * file address space page cache. This is possible if no one else has
+ * the pipe page referenced outside of the pipe and page cache. If
+ * SPLICE_F_MOVE isn't set, or we cannot move the page, we simply create
+ * a new page in the output file page cache and fill/dirty that.
*/
static int pipe_to_file(struct pipe_inode_info *info, struct pipe_buffer *buf,
struct splice_desc *sd)
@@ -401,6 +423,10 @@
* reuse buf page, if SPLICE_F_MOVE is set
*/
if (sd->flags & SPLICE_F_MOVE) {
+ /*
+ * If steal succeeds, buf->page is now pruned from the vm
+ * side (LRU and page cache) and we can reuse it.
+ */
if (buf->ops->steal(info, buf))
goto find_page;
@@ -484,6 +510,11 @@
typedef int (splice_actor)(struct pipe_inode_info *, struct pipe_buffer *,
struct splice_desc *);
+/*
+ * Pipe input worker. Most of this logic works like a regular pipe, the
+ * key here is the 'actor' worker passed in that actually moves the data
+ * to the wanted destination. See pipe_to_file/pipe_to_sendpage above.
+ */
static ssize_t move_from_pipe(struct inode *inode, struct file *out,
size_t len, unsigned int flags,
splice_actor *actor)
@@ -585,6 +616,17 @@
}
+/**
+ * generic_file_splice_write - splice data from a pipe to a file
+ * @inode: pipe inode
+ * @out: file to write to
+ * @len: number of bytes to splice
+ * @flags: splice modifier flags
+ *
+ * Will either move or copy pages (determined by @flags options) from
+ * the given pipe inode to the given file.
+ *
+ */
ssize_t generic_file_splice_write(struct inode *inode, struct file *out,
size_t len, unsigned int flags)
{
@@ -611,6 +653,17 @@
return ret;
}
+/**
+ * generic_splice_sendpage - splice data from a pipe to a socket
+ * @inode: pipe inode
+ * @out: socket to write to
+ * @len: number of bytes to splice
+ * @flags: splice modifier flags
+ *
+ * Will send @len bytes from the pipe to a network socket. No data copying
+ * is involved.
+ *
+ */
ssize_t generic_splice_sendpage(struct inode *inode, struct file *out,
size_t len, unsigned int flags)
{
@@ -620,6 +673,9 @@
EXPORT_SYMBOL(generic_file_splice_write);
EXPORT_SYMBOL(generic_file_splice_read);
+/*
+ * Attempt to initiate a splice from pipe to file.
+ */
static long do_splice_from(struct inode *pipe, struct file *out, size_t len,
unsigned int flags)
{
@@ -640,6 +696,9 @@
return out->f_op->splice_write(pipe, out, len, flags);
}
+/*
+ * Attempt to initiate a splice from a file to a pipe.
+ */
static long do_splice_to(struct file *in, struct inode *pipe, size_t len,
unsigned int flags)
{
@@ -668,6 +727,9 @@
return in->f_op->splice_read(in, pipe, len, flags);
}
+/*
+ * Determine where to splice to/from.
+ */
static long do_splice(struct file *in, struct file *out, size_t len,
unsigned int flags)
{