Re: [PATCH v4] splice: only read in as much information as there is pipe buffer space

Andreas Gruenbacher
2019-11-21T17:58:11Z

On Sat, Oct 19, 2019 at 6:14 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> Andreas Grünbacher reports that on the two filesystems that support
> iomap directio, it's possible for splice() to return -EAGAIN (instead of
> a short splice) if the pipe being written to has less space available in
> its pipe buffers than the length supplied by the calling process.
>
> Months ago we fixed splice_direct_to_actor to clamp the length of the
> read request to the size of the splice pipe. Do the same to do_splice.
>
> Fixes: 17614445576b6 ("splice: don't read more than available pipe space")
> Reported-by: Andreas Grünbacher <andreas.gruenbacher@gmail.com>

Reviewed-by: Andreas Grünbacher <andreas.gruenbacher@gmail.com>

> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> v4: use size_t for pipe_pages
> ---
> fs/splice.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/fs/splice.c b/fs/splice.c
> index 98412721f056..9b9b22d2215a 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -945,12 +945,13 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
> WARN_ON_ONCE(pipe->nrbufs != 0);
>
> while (len) {
> + size_t pipe_pages;
> size_t read_len;
> loff_t pos = sd->pos, prev_pos = pos;
>
> /* Don't try to read more the pipe has space for. */
> - read_len = min_t(size_t, len,
> - (pipe->buffers - pipe->nrbufs) << PAGE_SHIFT);
> + pipe_pages = pipe->buffers - pipe->nrbufs;
> + read_len = min(len, pipe_pages << PAGE_SHIFT);
> ret = do_splice_to(in, &pos, pipe, read_len, flags);
> if (unlikely(ret <= 0))
> goto out_release;
> @@ -1180,8 +1181,15 @@ static long do_splice(struct file *in, loff_t __user *off_in,
>
> pipe_lock(opipe);
> ret = wait_for_space(opipe, flags);
> - if (!ret)
> + if (!ret) {
> + size_t pipe_pages;
> +
> + /* Don't try to read more the pipe has space for. */
> + pipe_pages = opipe->buffers - opipe->nrbufs;
> + len = min(len, pipe_pages << PAGE_SHIFT);
> +
> ret = do_splice_to(in, &offset, opipe, len, flags);
> + }
> pipe_unlock(opipe);
> if (ret > 0)
> wakeup_pipe_readers(opipe);