[PATCH 3/3] iov_iter: import single vector iovecs as ITER_UBUF

[Jens Axboe <axboe@kernel.dk>]

Add a special case to __import_iovec(), which imports a single segment
iovec as an ITER_UBUF rather than an ITER_IOVEC. ITER_UBUF is cheaper
to iterate than ITER_IOVEC, and for a single segment iovec, there's no
point in using a segmented iterator.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 lib/iov_iter.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index fc82cc42ffe6..63cf9997bd50 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1780,6 +1780,30 @@ struct iovec *iovec_from_user(const struct iovec __user *uvec,
 	return iov;
 }
 
+/*
+ * Single segment iovec supplied by the user, import it as ITER_UBUF.
+ */
+static ssize_t __import_iovec_ubuf(int type, const struct iovec __user *uvec,
+				   struct iovec **iovp, struct iov_iter *i,
+				   bool compat)
+{
+	struct iovec *iov = *iovp;
+	ssize_t ret;
+
+	if (compat)
+		ret = copy_compat_iovec_from_user(iov, uvec, 1);
+	else
+		ret = copy_iovec_from_user(iov, uvec, 1);
+	if (unlikely(ret))
+		return ret;
+
+	ret = import_ubuf(type, iov->iov_base, iov->iov_len, i);
+	if (unlikely(ret))
+		return ret;
+	*iovp = NULL;
+	return i->count;
+}
+
 ssize_t __import_iovec(int type, const struct iovec __user *uvec,
 		 unsigned nr_segs, unsigned fast_segs, struct iovec **iovp,
 		 struct iov_iter *i, bool compat)
@@ -1788,6 +1812,9 @@ ssize_t __import_iovec(int type, const struct iovec __user *uvec,
 	unsigned long seg;
 	struct iovec *iov;
 
+	if (nr_segs == 1)
+		return __import_iovec_ubuf(type, uvec, iovp, i, compat);
+
 	iov = iovec_from_user(uvec, nr_segs, fast_segs, *iovp, compat);
 	if (IS_ERR(iov)) {
 		*iovp = NULL;
-- 
2.39.2