io_uring: follow **iovec idiom in io_import_iovec

As for import_iovec(), return !=NULL iovec from io_import_iovec() only
when it should be freed. That includes returning NULL when iovec is
already in req->io, because it should be deallocated by other means,
e.g. inside op handler. After io_setup_async_rw() local iovec to ->io,
just mark it NULL, to follow the idea in io_{read,write} as well.

That's easier to follow, and especially useful if we want to reuse
per-op space for completion data.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
[axboe: only call kfree() on non-NULL pointer]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2740,9 +2740,7 @@ static ssize_t io_import_iovec(int rw, struct io_kiocb *req,
 	if (req->io) {
 		struct io_async_rw *iorw = &req->io->rw;
 
-		*iovec = iorw->iov;
-		iov_iter_init(iter, rw, *iovec, iorw->nr_segs, iorw->size);
-		if (iorw->iov == iorw->fast_iov)
+		iov_iter_init(iter, rw, iorw->iov, iorw->nr_segs, iorw->size);
 		*iovec = NULL;
 		return iorw->size;
 	}
@@ -3026,6 +3024,8 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
 						inline_vecs, &iter);
 			if (ret)
 				goto out_free;
+			/* it's copied and will be cleaned with ->io */
+			iovec = NULL;
 			/* if we can retry, do so with the callbacks armed */
 			if (io_rw_should_retry(req)) {
 				ret2 = io_iter_do_read(req, &iter);
@@ -3041,7 +3041,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
 		}
 	}
 out_free:
-	if (!(req->flags & REQ_F_NEED_CLEANUP))
+	if (iovec)
 		kfree(iovec);
 	return ret;
 }
@@ -3143,11 +3143,13 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
 						inline_vecs, &iter);
 			if (ret)
 				goto out_free;
+			/* it's copied and will be cleaned with ->io */
+			iovec = NULL;
 			return -EAGAIN;
 		}
 	}
 out_free:
-	if (!(req->flags & REQ_F_NEED_CLEANUP))
+	if (iovec)
 		kfree(iovec);
 	return ret;
 }