Re: [PATCH v2] splice: only read in as much information as there is pipe buffer space

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

On Fri, Aug 30, 2019 at 02:06:03PM -0700, Darrick J. Wong 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>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> v2: tidy up the other call site per Andreas' request

Ping?  Anyone want to add a RVB to this?

--D

> ---
>  fs/splice.c |   17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 98412721f056..2ddbace9129f 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) {
> +		unsigned int 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, (size_t)pipe_pages << PAGE_SHIFT);
>  		ret = do_splice_to(in, &pos, pipe, read_len, flags);
>  		if (unlikely(ret <= 0))
>  			goto out_release;
> @@ -1101,6 +1102,7 @@ static long do_splice(struct file *in, loff_t __user *off_in,
>  	struct pipe_inode_info *ipipe;
>  	struct pipe_inode_info *opipe;
>  	loff_t offset;
> +	unsigned int pipe_pages;
>  	long ret;
>  
>  	ipipe = get_pipe_info(in);
> @@ -1123,6 +1125,10 @@ static long do_splice(struct file *in, loff_t __user *off_in,
>  		if ((in->f_flags | out->f_flags) & O_NONBLOCK)
>  			flags |= SPLICE_F_NONBLOCK;
>  
> +		/* Don't try to read more the pipe has space for. */
> +		pipe_pages = opipe->buffers - opipe->nrbufs;
> +		len = min(len, (size_t)pipe_pages << PAGE_SHIFT);
> +
>  		return splice_pipe_to_pipe(ipipe, opipe, len, flags);
>  	}
>  
> @@ -1180,8 +1186,13 @@ 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) {
> +			/* Don't try to read more the pipe has space for. */
> +			pipe_pages = opipe->buffers - opipe->nrbufs;
> +			len = min(len, (size_t)pipe_pages << PAGE_SHIFT);
> +
>  			ret = do_splice_to(in, &offset, opipe, len, flags);
> +		}
>  		pipe_unlock(opipe);
>  		if (ret > 0)
>  			wakeup_pipe_readers(opipe);