reactor: print more informative error when io_submit fails

when io_submit() fails with errno of ENOTSUP, there are good chances
that the application is sending i/o requests to a network filesystem
or a stacked filesystem. instead of printing

> seastar - io_submit: Operation not supported

let's print

> seastar - could not submit io. this happens when accessing filesystem which does not support asynchronous direct I/O

Refs #1947
Signed-off-by: Kefu Chai <[email protected]>

Closes #1949

src/core/reactor_backend.cc

@@ -179,6 +179,18 @@ aio_storage_context::handle_aio_error(linux_abi::iocb* iocb, int ec) {
             // we will only remove it from _pending_io and try again.
             return 1;
         }
+        case EINVAL:
+            // happens when the filesystem does not implement aio read or write
+            [[fallthrough]];
+        case ENOTSUP: {
+            seastar_logger.error("io_submit failed: this happens when "
+                                 "accessing filesystem which does not supports "
+                                 "asynchronous direct I/O");
+            auto desc = get_user_data<kernel_completion>(*iocb);
+            _iocb_pool.put_one(iocb);
+            desc->complete_with(-ENOTSUP);
+            return 1;
+        }
         default:
             ++_r._io_stats.aio_errors;
             throw std::system_error(ec, std::system_category(), "io_submit");