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

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Nov 21, 2019 at 06:57:55PM +0100, Andreas Gruenbacher wrote:
> 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>

Cool, thanks.  I'll try to push this to Linus next week.

--D

> > 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);
>