ITER_PIPE: clean iov_iter_revert()

Fold pipe_truncate() into it, clean up.  We can release buffers
in the same loop where we walk backwards to the iterator beginning
looking for the place where the new position will be.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -816,32 +816,6 @@ size_t copy_page_from_iter_atomic(struct page *page, unsigned offset, size_t byt
 }
 EXPORT_SYMBOL(copy_page_from_iter_atomic);
 
-static inline void pipe_truncate(struct iov_iter *i)
-{
-	struct pipe_inode_info *pipe = i->pipe;
-	unsigned int p_tail = pipe->tail;
-	unsigned int p_head = pipe->head;
-	unsigned int p_mask = pipe->ring_size - 1;
-
-	if (!pipe_empty(p_head, p_tail)) {
-		struct pipe_buffer *buf;
-		unsigned int i_head = i->head;
-		size_t off = i->iov_offset;
-
-		if (off) {
-			buf = &pipe->bufs[i_head & p_mask];
-			buf->len = off - buf->offset;
-			i_head++;
-		}
-		while (p_head != i_head) {
-			p_head--;
-			pipe_buf_release(pipe, &pipe->bufs[p_head & p_mask]);
-		}
-
-		pipe->head = p_head;
-	}
-}
-
 static void pipe_advance(struct iov_iter *i, size_t size)
 {
 	struct pipe_inode_info *pipe = i->pipe;
@@ -936,28 +910,22 @@ void iov_iter_revert(struct iov_iter *i, size_t unroll)
 	i->count += unroll;
 	if (unlikely(iov_iter_is_pipe(i))) {
 		struct pipe_inode_info *pipe = i->pipe;
-		unsigned int p_mask = pipe->ring_size - 1;
-		unsigned int i_head = i->head;
-		size_t off = i->iov_offset;
-		while (1) {
-			struct pipe_buffer *b = &pipe->bufs[i_head & p_mask];
-			size_t n = off - b->offset;
-			if (unroll < n) {
-				off -= unroll;
-				break;
-			}
-			unroll -= n;
-			if (!unroll && i_head == i->start_head) {
-				off = 0;
-				break;
+		unsigned int head = pipe->head;
+
+		while (head > i->start_head) {
+			struct pipe_buffer *b = pipe_buf(pipe, --head);
+			if (unroll < b->len) {
+				b->len -= unroll;
+				i->iov_offset = b->offset + b->len;
+				i->head = head;
+				return;
 			}
-			i_head--;
-			b = &pipe->bufs[i_head & p_mask];
-			off = b->offset + b->len;
+			unroll -= b->len;
+			pipe_buf_release(pipe, b);
+			pipe->head--;
 		}
-		i->iov_offset = off;
-		i->head = i_head;
-		pipe_truncate(i);
+		i->iov_offset = 0;
+		i->head = head;
 		return;
 	}
 	if (unlikely(iov_iter_is_discard(i)))