Re: [PATCH v14 08/17] splice: Do splice read from a file without using ITER_PIPE

Ming Lei
2023-02-17T08:23:40Z

On Tue, Feb 14, 2023 at 05:13:21PM +0000, David Howells wrote:
> Make generic_file_splice_read() use filemap_splice_read() and
> direct_splice_read() rather than using an ITER_PIPE and call_read_iter().
>
> With this, ITER_PIPE is no longer used.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Christoph Hellwig <hch@lst.de>
> cc: Al Viro <viro@zeniv.linux.org.uk>
> cc: David Hildenbrand <david@redhat.com>
> cc: John Hubbard <jhubbard@nvidia.com>
> cc: linux-mm@kvack.org
> cc: linux-block@vger.kernel.org
> cc: linux-fsdevel@vger.kernel.org
> ---
>
> Notes:
> ver #14)
> - Split out filemap_splice_read() into a separate patch.
>
> fs/splice.c | 30 +++++++-----------------------
> 1 file changed, 7 insertions(+), 23 deletions(-)
>
> diff --git a/fs/splice.c b/fs/splice.c
> index 4c6332854b63..a93478338cec 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -391,29 +391,13 @@ ssize_t generic_file_splice_read(struct file *in, loff_t *ppos,
> struct pipe_inode_info *pipe, size_t len,
> unsigned int flags)
> {
> - struct iov_iter to;
> - struct kiocb kiocb;
> - int ret;
> -
> - iov_iter_pipe(&to, ITER_DEST, pipe, len);
> - init_sync_kiocb(&kiocb, in);
> - kiocb.ki_pos = *ppos;
> - ret = call_read_iter(in, &kiocb, &to);
> - if (ret > 0) {
> - *ppos = kiocb.ki_pos;
> - file_accessed(in);
> - } else if (ret < 0) {
> - /* free what was emitted */
> - pipe_discard_from(pipe, to.start_head);
> - /*
> - * callers of ->splice_read() expect -EAGAIN on
> - * "can't put anything in there", rather than -EFAULT.
> - */
> - if (ret == -EFAULT)
> - ret = -EAGAIN;
> - }
> -
> - return ret;
> + if (unlikely(*ppos >= file_inode(in)->i_sb->s_maxbytes))
> + return 0;
> + if (unlikely(!len))
> + return 0;
> + if (in->f_flags & O_DIRECT)
> + return direct_splice_read(in, ppos, pipe, len, flags);

Hello David,

I have one question, for dio, pages need to map to userspace
memory, but direct_splice_read() just allocates pages and not
see when the user mapping is setup, can you give one hint?

Thanks,
Ming