[RFC PATCH 01/18] iov_iter: Add function to see if buffer is all zeros

[David Howells <dhowells@redhat.com>]

Add a function to scan a buffer and indicate if all of the bytes contained
therein are zero.

Signed-off-by: David Howells <dhowells@redhat.com>
---
 include/linux/uio.h |  1 +
 lib/iov_iter.c      | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index ff81e5ccaef2..49de7b8a8890 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -264,6 +264,7 @@ static inline bool iov_iter_is_copy_mc(const struct iov_iter *i)
 #endif
 
 size_t iov_iter_zero(size_t bytes, struct iov_iter *);
+bool iov_iter_is_zero(const struct iov_iter *i, size_t count);
 bool iov_iter_is_aligned(const struct iov_iter *i, unsigned addr_mask,
 			unsigned len_mask);
 unsigned long iov_iter_alignment(const struct iov_iter *i);
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index b667b1e2f688..ec9e3e1a11a9 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -566,6 +566,28 @@ size_t iov_iter_zero(size_t bytes, struct iov_iter *i)
 }
 EXPORT_SYMBOL(iov_iter_zero);
 
+/**
+ * iov_iter_is_zero - Return true if the buffer is entirely zeroed
+ * @i: The iterator describing the buffer
+ * @count: Amount of buffer to scan
+ *
+ * Scans the specified amount of the supplied buffer and returns true if only
+ * zero bytes are found therein and false otherwise.
+ */
+bool iov_iter_is_zero(const struct iov_iter *i, size_t count)
+{
+	struct iov_iter j = *i, *pj = &j;
+	void *p;
+
+	iterate_and_advance(pj, count, base, len, count,
+			    ({ p = memchr_inv(base, 0, len); p ? p - base : len; }),
+			    ({ p = memchr_inv(base, 0, len); p ? p - base : len; })
+	)
+
+	return !count;
+}
+EXPORT_SYMBOL(iov_iter_is_zero);
+
 size_t copy_page_from_iter_atomic(struct page *page, unsigned offset, size_t bytes,
 				  struct iov_iter *i)
 {