io_uring/rsrc: separate kbuf offset adjustments

Kernel registered buffers are special because segments are not uniform
in size, and we have a bunch of optimisations based on that uniformity
for normal buffers. Handle kbuf separately, it'll be cleaner this way.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/4e9e5990b0ab5aee723c0be5cd9b5bcf810375f9.1744882081.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/rsrc.c

@@ -1048,11 +1048,14 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 	if (!(imu->dir & (1 << ddir)))
 		return -EFAULT;
 
-	/*
-	 * Might not be a start of buffer, set size appropriately
-	 * and advance us to the beginning.
-	 */
 	offset = buf_addr - imu->ubuf;
+
+	if (imu->is_kbuf) {
+		iov_iter_bvec(iter, ddir, imu->bvec, imu->nr_bvecs, offset + len);
+		iov_iter_advance(iter, offset);
+		return 0;
+	}
+
 	iov_iter_bvec(iter, ddir, imu->bvec, imu->nr_bvecs, offset + len);
 
 	/*
@@ -1072,17 +1075,9 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 	 * be folio_size aligned.
 	 */
 	bvec = imu->bvec;
-
-	/*
-	 * Kernel buffer bvecs, on the other hand, don't necessarily
-	 * have the size property of user registered ones, so we have
-	 * to use the slow iter advance.
-	 */
 	if (offset < bvec->bv_len) {
 		iter->count -= offset;
 		iter->iov_offset = offset;
-	} else if (imu->is_kbuf) {
-		iov_iter_advance(iter, offset);
 	} else {
 		unsigned long seg_skip;