1 files changed, 66 insertions, 4 deletions

diff --git a/fs/splice.c b/fs/splice.c
index 07f4d863c2d4..34591924c783 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -37,6 +37,12 @@ struct splice_desc {
         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 @@ static struct pipe_buf_operations page_cache_pipe_buf_ops = {
         .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 @@ splice_them:
         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 @@ static int pipe_to_sendpage(struct pipe_inode_info *info,
  *      - 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 @@ static int pipe_to_file(struct pipe_inode_info *info, struct pipe_buffer *buf,
          * 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 @@ out:
 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 @@ static ssize_t move_from_pipe(struct inode *inode, struct file *out,
 
 }
 
+/**
+ * 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 @@ ssize_t generic_file_splice_write(struct inode *inode, struct file *out,
         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 @@ ssize_t generic_splice_sendpage(struct inode *inode, struct file *out,
 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 @@ static long do_splice_from(struct inode *pipe, struct file *out, size_t len,
         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 @@ static long do_splice_to(struct file *in, struct inode *pipe, size_t len,
         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)
 {