[PATCH v3 3/6] SUNRPC: Introduce xdr_buf_trim_head()

[Anna Schumaker <anna@kernel.org>]

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

The READ_PLUS operation uses a 32-bit length field for encoding a DATA
segment, but 64-bit length field for encoding a HOLE segment. When
setting up our reply buffer, we need to reserve enough space to encode
a HOLE before reading the file data and use this function if the first
segment turns out to be DATA.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
---
 include/linux/sunrpc/xdr.h |  1 +
 net/sunrpc/xdr.c           | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
index e26047d474b2..bdaf048edde0 100644
--- a/include/linux/sunrpc/xdr.h
+++ b/include/linux/sunrpc/xdr.h
@@ -191,6 +191,7 @@ xdr_adjust_iovec(struct kvec *iov, __be32 *p)
 extern void xdr_shift_buf(struct xdr_buf *, size_t);
 extern void xdr_buf_from_iov(const struct kvec *, struct xdr_buf *);
 extern int xdr_buf_subsegment(const struct xdr_buf *, struct xdr_buf *, unsigned int, unsigned int);
+extern void xdr_buf_trim_head(struct xdr_buf *, unsigned int);
 extern void xdr_buf_trim(struct xdr_buf *, unsigned int);
 extern int read_bytes_from_xdr_buf(const struct xdr_buf *, unsigned int, void *, unsigned int);
 extern int write_bytes_to_xdr_buf(const struct xdr_buf *, unsigned int, void *, unsigned int);
diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
index 63d9cdc989da..37956a274f81 100644
--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -1739,6 +1739,23 @@ unsigned int xdr_stream_move_subsegment(struct xdr_stream *xdr, unsigned int off
 }
 EXPORT_SYMBOL_GPL(xdr_stream_move_subsegment);
 
+/**
+ * xdr_buf_trim_head - lop at most "len" bytes off the end of "buf"->head
+ * @buf: buf to be trimmed
+ * @len: number of bytes to reduce "buf"->head by
+ *
+ * Trim an xdr_buf->head by the given number of bytes by fixing up the lengths.
+ * Note that it's possible that we'll trim less than that amount if the
+ *  xdr_buf->head is too small.
+ */
+void xdr_buf_trim_head(struct xdr_buf *buf, unsigned int len)
+{
+	size_t trim = min_t(size_t, buf->head[0].iov_len, len);
+	buf->head[0].iov_len -= trim;
+	buf->len -= trim;
+}
+EXPORT_SYMBOL_GPL(xdr_buf_trim_head);
+
 /**
  * xdr_buf_trim - lop at most "len" bytes off the end of "buf"
  * @buf: buf to be trimmed
-- 
2.37.1