io_uring/rsrc: refactor io_import_fixed

io_import_fixed is a mess. Even though we know the final len of the
iterator, we still assign offset + len and do some magic after to
correct for that.

Do offset calculation first and finalise it with iov_iter_bvec at the
end.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2d5107fed24f8b23245ef2ede9a5a7f7c426df61.1744882081.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/rsrc.c

@@ -1037,6 +1037,7 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 			   u64 buf_addr, size_t len)
 {
 	const struct bio_vec *bvec;
+	unsigned nr_segs;
 	size_t offset;
 	int ret;
 
@@ -1056,8 +1057,6 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 		return 0;
 	}
 
-	iov_iter_bvec(iter, ddir, imu->bvec, imu->nr_bvecs, offset + len);
-
 	/*
 	 * Don't use iov_iter_advance() here, as it's really slow for
 	 * using the latter parts of a big fixed buffer - it iterates
@@ -1067,30 +1066,21 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 	 * 1) it's a BVEC iter, we set it up
 	 * 2) all bvecs are the same in size, except potentially the
 	 *    first and last bvec
-	 *
-	 * So just find our index, and adjust the iterator afterwards.
-	 * If the offset is within the first bvec (or the whole first
-	 * bvec, just use iov_iter_advance(). This makes it easier
-	 * since we can just skip the first segment, which may not
-	 * be folio_size aligned.
 	 */
 	bvec = imu->bvec;
-	if (offset < bvec->bv_len) {
-		iter->count -= offset;
-		iter->iov_offset = offset;
-	} else {
+	if (offset >= bvec->bv_len) {
 		unsigned long seg_skip;
 
 		/* skip first vec */
 		offset -= bvec->bv_len;
 		seg_skip = 1 + (offset >> imu->folio_shift);
-
-		iter->bvec += seg_skip;
-		iter->nr_segs -= seg_skip;
-		iter->count -= bvec->bv_len + offset;
-		iter->iov_offset = offset & ((1UL << imu->folio_shift) - 1);
+		bvec += seg_skip;
+		offset &= (1UL << imu->folio_shift) - 1;
 	}
 
+	nr_segs = imu->nr_bvecs - (bvec - imu->bvec);
+	iov_iter_bvec(iter, ddir, bvec, nr_segs, len);
+	iter->iov_offset = offset;
 	return 0;
 }