ITER_PIPE: helper for getting pipe buffer by index

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...

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index b3493d20536e279073ac19b3b9a61a310eb156c5..048026d5aa0d2e36ddf2e7e5e43c21f0da55f857 100644 (file)
--- 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;
@@ -1280,10 +1284,9 @@ 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;
        }