[PATCH for-next 3/4] io_uring/rsrc: optimise single entry advance

[Pavel Begunkov <asml.silence@gmail.com>]

Iterating within the first bvec entry should be essentially free, but we
use iov_iter_advance() for that, which shows up in benchmark profiles
taking up to 0.5% of CPU. Replace it with a hand coded version.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/rsrc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 53845e496881..ebbd2cea7582 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -1364,7 +1364,10 @@ int io_import_fixed(int ddir, struct iov_iter *iter,
 		const struct bio_vec *bvec = imu->bvec;
 
 		if (offset <= bvec->bv_len) {
-			iov_iter_advance(iter, offset);
+			iter->bvec = bvec;
+			iter->nr_segs = bvec->bv_len;
+			iter->count -= offset;
+			iter->iov_offset = offset;
 		} else {
 			unsigned long seg_skip;
 
-- 
2.39.1