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SubjectRe: [PATCH] splice support #2
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

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