On Wed, 2022-06-22 at 05:15 +0100, Al Viro wrote:
> pipe_buffer instances of a pipe are organized as a ring buffer,
> with power-of-2 size. Indices are kept *not* reduced modulo ring
> size, so the buffer refered to by index N is
> pipe->bufs[N & (pipe->ring_size - 1)].
>
> Ring size can change over the lifetime of a pipe, but not while
> the pipe is locked. So for any iov_iter primitives it's a constant.
> Original conversion of pipes to this layout went overboard trying
> to microoptimize that - calculating pipe->ring_size - 1, storing
> it in a local variable and using through the function. In some
> cases it might be warranted, but most of the times it only
> obfuscates what's going on in there.
>
> Introduce a helper (pipe_buf(pipe, N)) that would encapsulate
> that and use it in the obvious cases. More will follow...
>
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
> lib/iov_iter.c | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index d00cc8971b5b..08bb393da677 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -183,13 +183,18 @@ static int copyin(void *to, const void __user *from, size_t n)
> return n;
> }
>
> +static inline struct pipe_buffer *pipe_buf(const struct pipe_inode_info *pipe,
> + unsigned int slot)
> +{
> + return &pipe->bufs[slot & (pipe->ring_size - 1)];
> +}
> +
> #ifdef PIPE_PARANOIA
> static bool sanity(const struct iov_iter *i)
> {
> struct pipe_inode_info *pipe = i->pipe;
> unsigned int p_head = pipe->head;
> unsigned int p_tail = pipe->tail;
> - unsigned int p_mask = pipe->ring_size - 1;
> unsigned int p_occupancy = pipe_occupancy(p_head, p_tail);
> unsigned int i_head = i->head;
> unsigned int idx;
> @@ -201,7 +206,7 @@ static bool sanity(const struct iov_iter *i)
> if (unlikely(i_head != p_head - 1))
> goto Bad; // must be at the last buffer...
>
> - p = &pipe->bufs[i_head & p_mask];
> + p = pipe_buf(pipe, i_head);
> if (unlikely(p->offset + p->len != i->iov_offset))
> goto Bad; // ... at the end of segment
> } else {
> @@ -386,11 +391,10 @@ static inline bool allocated(struct pipe_buffer *buf)
> static inline void data_start(const struct iov_iter *i,
> unsigned int *iter_headp, size_t *offp)
> {
> - unsigned int p_mask = i->pipe->ring_size - 1;
> unsigned int iter_head = i->head;
> size_t off = i->iov_offset;
>
> - if (off && (!allocated(&i->pipe->bufs[iter_head & p_mask]) ||
> + if (off && (!allocated(pipe_buf(i->pipe, iter_head)) ||
> off == PAGE_SIZE)) {
> iter_head++;
> off = 0;
> @@ -1180,10 +1184,9 @@ unsigned long iov_iter_alignment(const struct iov_iter *i)
> return iov_iter_alignment_bvec(i);
>
> if (iov_iter_is_pipe(i)) {
> - unsigned int p_mask = i->pipe->ring_size - 1;
> size_t size = i->count;
>
> - if (size && i->iov_offset && allocated(&i->pipe->bufs[i->head & p_mask]))
> + if (size && i->iov_offset && allocated(pipe_buf(i->pipe, i->head)))
> return size | i->iov_offset;
> return size;
> }
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
