[PATCH v3 4/6] SUNRPC: Introduce xdr_buf_nth_page_address()

[Anna Schumaker <anna@kernel.org>]

From: Anna Schumaker <Anna.Schumaker@Netapp.com>

For getting a pointer to the memory address represented by the nth page,
along with the length of the data on that page.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
---
 include/linux/sunrpc/xdr.h |  2 ++
 net/sunrpc/xdr.c           | 17 +++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
index bdaf048edde0..79824fea4529 100644
--- a/include/linux/sunrpc/xdr.h
+++ b/include/linux/sunrpc/xdr.h
@@ -137,6 +137,8 @@ void	xdr_inline_pages(struct xdr_buf *, unsigned int,
 			 struct page **, unsigned int, unsigned int);
 void	xdr_terminate_string(const struct xdr_buf *, const u32);
 size_t	xdr_buf_pagecount(const struct xdr_buf *buf);
+char	*xdr_buf_nth_page_address(const struct xdr_buf *buf, unsigned int n,
+				  unsigned int *len);
 int	xdr_alloc_bvec(struct xdr_buf *buf, gfp_t gfp);
 void	xdr_free_bvec(struct xdr_buf *buf);
 
diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
index 37956a274f81..88b28656a05d 100644
--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -140,6 +140,23 @@ size_t xdr_buf_pagecount(const struct xdr_buf *buf)
 	return (buf->page_base + buf->page_len + PAGE_SIZE - 1) >> PAGE_SHIFT;
 }
 
+char *xdr_buf_nth_page_address(const struct xdr_buf *buf, unsigned int n,
+			       unsigned int *len)
+{
+	unsigned int pgbase = buf->page_base + (n * PAGE_SIZE);
+	struct page **pages = buf->pages;
+	struct page **page;
+
+	if (n >= xdr_buf_pagecount(buf))
+		return NULL;
+
+	page = pages + (pgbase >> PAGE_SHIFT);
+	pgbase &= ~PAGE_MASK;
+	*len = min_t(size_t, PAGE_SIZE, buf->page_len - (n * PAGE_SIZE));
+	return page_address(*page) + pgbase;
+}
+EXPORT_SYMBOL_GPL(xdr_buf_nth_page_address);
+
 int
 xdr_alloc_bvec(struct xdr_buf *buf, gfp_t gfp)
 {
-- 
2.37.1