Re: [RFC PATCH 04/10] pipe: Use head and tail pointers for the ring, not cursor and length [ver #2]

[Ilya Dryomov <idryomov@gmail.com>]

On Thu, Oct 24, 2019 at 11:49 AM David Howells <dhowells@redhat.com> wrote:
>
> Convert pipes to use head and tail pointers for the buffer ring rather than
> pointer and length as the latter requires two atomic ops to update (or a
> combined op) whereas the former only requires one.
>
>  (1) The head pointer is the point at which production occurs and points to
>      the slot in which the next buffer will be placed.  This is equivalent
>      to pipe->curbuf + pipe->nrbufs.
>
>      The head pointer belongs to the write-side.
>
>  (2) The tail pointer is the point at which consumption occurs.  It points
>      to the next slot to be consumed.  This is equivalent to pipe->curbuf.
>
>      The tail pointer belongs to the read-side.
>
>  (3) head and tail are allowed to run to UINT_MAX and wrap naturally.  They
>      are only masked off when the array is being accessed, e.g.:
>
>         pipe->bufs[head & mask]
>
>      This means that it is not necessary to have a dead slot in the ring as
>      head == tail isn't ambiguous.
>
>  (4) The ring is empty if "head == tail".
>
>      A helper, pipe_empty(), is provided for this.
>
>  (5) The occupancy of the ring is "head - tail".
>
>      A helper, pipe_occupancy(), is provided for this.
>
>  (6) The number of free slots in the ring is "pipe->ring_size - occupancy".
>
>      A helper, pipe_space_for_user() is provided to indicate how many slots
>      userspace may use.
>
>  (7) The ring is full if "head - tail >= pipe->ring_size".
>
>      A helper, pipe_full(), is provided for this.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
>
>  fs/fuse/dev.c             |   31 +++--
>  fs/pipe.c                 |  169 ++++++++++++++++-------------
>  fs/splice.c               |  188 ++++++++++++++++++++------------
>  include/linux/pipe_fs_i.h |   86 ++++++++++++++-
>  include/linux/uio.h       |    4 -
>  lib/iov_iter.c            |  266 +++++++++++++++++++++++++--------------------
>  6 files changed, 464 insertions(+), 280 deletions(-)
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index dadd617d826c..1e4bc27573cc 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -703,7 +703,7 @@ static int fuse_copy_fill(struct fuse_copy_state *cs)
>                         cs->pipebufs++;
>                         cs->nr_segs--;
>                 } else {
> -                       if (cs->nr_segs == cs->pipe->buffers)
> +                       if (cs->nr_segs >= cs->pipe->ring_size)
>                                 return -EIO;
>
>                         page = alloc_page(GFP_HIGHUSER);
> @@ -879,7 +879,7 @@ static int fuse_ref_page(struct fuse_copy_state *cs, struct page *page,
>         struct pipe_buffer *buf;
>         int err;
>
> -       if (cs->nr_segs == cs->pipe->buffers)
> +       if (cs->nr_segs >= cs->pipe->ring_size)
>                 return -EIO;
>
>         err = unlock_request(cs->req);
> @@ -1341,7 +1341,7 @@ static ssize_t fuse_dev_splice_read(struct file *in, loff_t *ppos,
>         if (!fud)
>                 return -EPERM;
>
> -       bufs = kvmalloc_array(pipe->buffers, sizeof(struct pipe_buffer),
> +       bufs = kvmalloc_array(pipe->ring_size, sizeof(struct pipe_buffer),
>                               GFP_KERNEL);
>         if (!bufs)
>                 return -ENOMEM;
> @@ -1353,7 +1353,7 @@ static ssize_t fuse_dev_splice_read(struct file *in, loff_t *ppos,
>         if (ret < 0)
>                 goto out;
>
> -       if (pipe->nrbufs + cs.nr_segs > pipe->buffers) {
> +       if (pipe_occupancy(pipe->head, pipe->tail) + cs.nr_segs > pipe->ring_size) {
>                 ret = -EIO;
>                 goto out;
>         }
> @@ -1935,6 +1935,7 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
>                                      struct file *out, loff_t *ppos,
>                                      size_t len, unsigned int flags)
>  {
> +       unsigned int head, tail, mask, count;
>         unsigned nbuf;
>         unsigned idx;
>         struct pipe_buffer *bufs;
> @@ -1949,8 +1950,12 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
>
>         pipe_lock(pipe);
>
> -       bufs = kvmalloc_array(pipe->nrbufs, sizeof(struct pipe_buffer),
> -                             GFP_KERNEL);
> +       head = pipe->head;
> +       tail = pipe->tail;
> +       mask = pipe->ring_size - 1;
> +       count = head - tail;
> +
> +       bufs = kvmalloc_array(count, sizeof(struct pipe_buffer), GFP_KERNEL);
>         if (!bufs) {
>                 pipe_unlock(pipe);
>                 return -ENOMEM;
> @@ -1958,8 +1963,8 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
>
>         nbuf = 0;
>         rem = 0;
> -       for (idx = 0; idx < pipe->nrbufs && rem < len; idx++)
> -               rem += pipe->bufs[(pipe->curbuf + idx) & (pipe->buffers - 1)].len;
> +       for (idx = tail; idx < head && rem < len; idx++)
> +               rem += pipe->bufs[idx & mask].len;
>
>         ret = -EINVAL;
>         if (rem < len)
> @@ -1970,16 +1975,16 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
>                 struct pipe_buffer *ibuf;
>                 struct pipe_buffer *obuf;
>
> -               BUG_ON(nbuf >= pipe->buffers);
> -               BUG_ON(!pipe->nrbufs);
> -               ibuf = &pipe->bufs[pipe->curbuf];
> +               BUG_ON(nbuf >= pipe->ring_size);
> +               BUG_ON(tail == head);
> +               ibuf = &pipe->bufs[tail & mask];
>                 obuf = &bufs[nbuf];
>
>                 if (rem >= ibuf->len) {
>                         *obuf = *ibuf;
>                         ibuf->ops = NULL;
> -                       pipe->curbuf = (pipe->curbuf + 1) & (pipe->buffers - 1);
> -                       pipe->nrbufs--;
> +                       tail++;
> +                       pipe_commit_read(pipe, tail);
>                 } else {
>                         if (!pipe_buf_get(pipe, ibuf))
>                                 goto out_free;
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 8a2ab2f974bd..8a0806fe12d3 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -43,10 +43,11 @@ unsigned long pipe_user_pages_hard;
>  unsigned long pipe_user_pages_soft = PIPE_DEF_BUFFERS * INR_OPEN_CUR;
>
>  /*
> - * We use a start+len construction, which provides full use of the
> - * allocated memory.
> - * -- Florian Coosmann (FGC)
> - *
> + * We use head and tail indices that aren't masked off, except at the point of
> + * dereference, but rather they're allowed to wrap naturally.  This means there
> + * isn't a dead spot in the buffer, provided the ring size < INT_MAX.
> + * -- David Howells 2019-09-23.

Hi David,

Is "ring size < INT_MAX" constraint correct?

I've never had to implement this free running indices scheme, but
the way I've always visualized it is that the top bit of the index is
used as a lap (as in a race) indicator, leaving 31 bits to work with
(in case of unsigned ints).  Should that be

  ring size <= 2^31

or more precisely

  ring size is a power of two <= 2^31

or am I missing something?

Thanks,

                Ilya