ITER_PIPE: clean pipe_advance() up

instead of setting ->iov_offset for new position and calling
pipe_truncate() to adjust ->len of the last buffer and discard
everything after it, adjust ->len at the same time we set ->iov_offset
and use pipe_discard_from() to deal with buffers past that.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -845,27 +845,27 @@ static inline void pipe_truncate(struct iov_iter *i)
 static void pipe_advance(struct iov_iter *i, size_t size)
 {
 	struct pipe_inode_info *pipe = i->pipe;
-	if (size) {
-		struct pipe_buffer *buf;
-		unsigned int p_mask = pipe->ring_size - 1;
-		unsigned int i_head = i->head;
-		size_t off = i->iov_offset, left = size;
+	unsigned int off = i->iov_offset;
 
-		if (off) /* make it relative to the beginning of buffer */
-			left += off - pipe->bufs[i_head & p_mask].offset;
+	if (!off && !size) {
+		pipe_discard_from(pipe, i->start_head); // discard everything
+		return;
+	}
+	i->count -= size;
 	while (1) {
-			buf = &pipe->bufs[i_head & p_mask];
-			if (left <= buf->len)
+		struct pipe_buffer *buf = pipe_buf(pipe, i->head);
+		if (off) /* make it relative to the beginning of buffer */
+			size += off - buf->offset;
+		if (size <= buf->len) {
+			buf->len = size;
+			i->iov_offset = buf->offset + size;
 			break;
-			left -= buf->len;
-			i_head++;
 		}
-		i->head = i_head;
-		i->iov_offset = buf->offset + left;
+		size -= buf->len;
+		i->head++;
+		off = 0;
 	}
-	i->count -= size;
-	/* ... and discard everything past that point */
-	pipe_truncate(i);
+	pipe_discard_from(pipe, i->head + 1); // discard everything past this one
 }
 
 static void iov_iter_bvec_advance(struct iov_iter *i, size_t size)