Re: [PATCH] splice support #2

[Jens Axboe <axboe@suse.de>]

On Fri, Mar 31 2006, Jens Axboe wrote:
> On Fri, Mar 31 2006, Ingo Molnar wrote:
> > 
> > * Linus Torvalds <torvalds@osdl.org> wrote:
> > 
> > >  - The pipe is the buffer #2: it's what allows you to do _other_ things 
> > >    with splice that are simply impossible to do with sendfile. Notably, 
> > >    splice allows very naturally the "readv/writev" scatter-gather 
> > >    behaviour of _mixing_ streams. If you're a web-server, with splice you 
> > >    can do
> > > 
> > > 	write(pipefd, header, header_len);
> > > 	splice(file, pipefd, file_len);
> > > 	splice(pipefd, socket, total_len);
> > > 
> > >    (this is all conceptual pseudo-code, of course), and this very 
> > >    naturally has none of the issues that sendfile() has with plugging etc. 
> > >    There's never any "send header separately and do extra work to make 
> > >    sure it is in the same packet as the start of the data".
> > 
> > with pipe-based buffering this approach has still the very same problems 
> > that sendfile() has with packet boundaries, because it's not enough to 
> > have "large enough" buffering (like a pipe has), the pipe also has to be 
> > drained, and the networking layer has to know the precise boundary of 
> > data.
> > 
> > the right solution to the packet boundary problem is to pass in a proper 
> > "does userspace expect more data right now" flag, or to let userspace 
> > 'flush' the socket independently - which is independent of the 
> > pipe-in-slice issue. This solution already exists: the MSG_MORE flag.
> 
> We can add a SPLICE_F_MORE flag for this, right now splice doesn't set
> the MSG_MORE flag for the end of the pipe.

Ala

diff --git a/fs/splice.c b/fs/splice.c
index d8787c1..8a9ef67 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -343,15 +343,16 @@ static int pipe_to_sendpage(struct pipe_
 	loff_t pos = sd->pos;
 	unsigned int offset;
 	ssize_t ret;
+	int more;
 
 	ret = buf->ops->map(file, info, buf);
 	if (unlikely(ret))
 		return ret;
 
 	offset = pos & ~PAGE_CACHE_MASK;
+	more = (sd->flags & SPLICE_F_MORE) || sd->len < sd->total_len;
 
-	ret = file->f_op->sendpage(file, buf->page, offset, sd->len, &pos,
-					sd->len < sd->total_len);
+	ret = file->f_op->sendpage(file, buf->page, offset, sd->len, &pos,more);
 
 	buf->ops->unmap(info, buf);
 	if (ret == sd->len)
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index bdf9d57..b003e3d 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -59,5 +59,6 @@ void free_pipe_info(struct inode* inode)
  * add the splice flags here.
  */
 #define SPLICE_F_MOVE	(0x01)	/* move pages instead of copying */
+#define SPLICE_F_MORE	(0x02)	/* expect more data */
 
 #endif

-- 
Jens Axboe