[PATCH v4 06/13] iov_iter: Add a kernel-type iterator-only iteration function

[David Howells <dhowells@redhat.com>]

Add an iteration function that can only iterate over kernel internal-type
iterators (ie. BVEC, KVEC, XARRAY) and not user-backed iterators (ie. UBUF
and IOVEC).  This allows for smaller iterators to be built when it is known
the caller won't have a user-backed iterator.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: Christoph Hellwig <hch@lst.de>
cc: Christian Brauner <christian@brauner.io>
cc: Matthew Wilcox <willy@infradead.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: David Laight <David.Laight@ACULAB.COM>
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
---
 include/linux/iov_iter.h | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/include/linux/iov_iter.h b/include/linux/iov_iter.h
index 836854847cdf..46e628640934 100644
--- a/include/linux/iov_iter.h
+++ b/include/linux/iov_iter.h
@@ -258,4 +258,39 @@ size_t iterate_and_advance(struct iov_iter *iter, size_t len, void *priv,
 	return iterate_and_advance2(iter, len, priv, NULL, ustep, step);
 }
 
+/**
+ * iterate_and_advance_kernel - Iterate over a kernel iterator
+ * @iter: The iterator to iterate over.
+ * @len: The amount to iterate over.
+ * @priv: Data for the step functions.
+ * @step: Processing function; given kernel addresses.
+ *
+ * Like iterate_and_advance2(), but rejected UBUF and IOVEC iterators and does
+ * not take a user-step function.
+ */
+static __always_inline
+size_t iterate_and_advance_kernel(struct iov_iter *iter, size_t len, void *priv,
+				  void *priv2, iov_step_f step)
+{
+	size_t progress;
+
+	if (unlikely(iter->count < len))
+		len = iter->count;
+	if (unlikely(!len))
+		return 0;
+
+	if (iov_iter_is_bvec(iter))
+		progress = iterate_bvec(iter, len, priv, priv2, step);
+	else if (iov_iter_is_kvec(iter))
+		progress = iterate_kvec(iter, len, priv, priv2, step);
+	else if (iov_iter_is_xarray(iter))
+		progress = iterate_xarray(iter, len, priv, priv2, step);
+	else if (WARN_ON_ONCE(unlikely(iter_is_ubuf(iter) || iter_is_iovec(iter))))
+		progress = 0;
+	else
+		progress = len;
+	iter->count -= progress;
+	return progress;
+}
+
 #endif /* _LINUX_IOV_ITER_H */