[PATCH] iov_iter: optimise bvec iov_iter_advance()

[Pavel Begunkov <asml.silence@xxxxxxxxx>]

iov_iter_advance() is heavily used, but implemented through generic
iteration. As bvecs have a specifically crafted advance() function, i.e.
bvec_iter_advance(), which is faster and slimmer, use it instead.

Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
 lib/iov_iter.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 1635111c5bd2..7fbdd9ef3ff0 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1077,6 +1077,20 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
 		i->count -= size;
 		return;
 	}
+	if (iov_iter_is_bvec(i)) {
+		struct bvec_iter bi;
+
+		bi.bi_size = i->count;
+		bi.bi_bvec_done = i->iov_offset;
+		bi.bi_idx = 0;
+		bvec_iter_advance(i->bvec, &bi, size);
+
+		i->bvec += bi.bi_idx;
+		i->nr_segs -= bi.bi_idx;
+		i->count = bi.bi_size;
+		i->iov_offset = bi.bi_bvec_done;
+		return;
+	}
 	iterate_and_advance(i, size, v, 0, 0, 0)
 }
 EXPORT_SYMBOL(iov_iter_advance);
-- 
2.24.0