往期文章：
spdk环境搭建

hello_bdev

代码路径：examples/bdev/hello_world/hello_bdev.c
可执行文件路径：build/examples/hello_bdev

刚开始直接执行hello_bdev显示找不到Malloc0

./build/examples/hello_bdev
[2023-05-30 20:27:02.389489] Starting SPDK v23.05-pre git sha1 ee06693c3 / DPDK 22.11.1 initialization...
[2023-05-30 20:27:02.390910] [ DPDK EAL parameters: hello_bdev --no-shconf -c 0x1 --huge-unlink --log-level=lib.eal:6 --log-level=lib.cryptodev:5 --log-level=user1:6 --iova-mode=pa --base-virtaddr=0x200000000000 --match-allocations --file-prefix=spdk_pid11584 ]
TELEMETRY: No legacy callbacks, legacy socket not created
[2023-05-30 20:27:02.511380] app.c: 738:spdk_app_start: *NOTICE*: Total cores available: 1
[2023-05-30 20:27:02.561201] reactor.c: 937:reactor_run: *NOTICE*: Reactor started on core 0
[2023-05-30 20:27:02.600284] accel_sw.c: 601:sw_accel_module_init: *NOTICE*: Accel framework software module initialized.
[2023-05-30 20:27:02.621229] hello_bdev.c: 222:hello_start: *NOTICE*: Successfully started the application
[2023-05-30 20:27:02.621612] hello_bdev.c: 231:hello_start: *NOTICE*: Opening the bdev Malloc0
[2023-05-30 20:27:02.621691] bdev.c:7681:spdk_bdev_open_ext: *NOTICE*: Currently unable to find bdev with name: Malloc0
[2023-05-30 20:27:02.621761] hello_bdev.c: 235:hello_start: *ERROR*: Could not open bdev: Malloc0
[2023-05-30 20:27:02.621852] app.c: 844:spdk_app_stop: *WARNING*: spdk_app_stop'd on non-zero
[2023-05-30 20:27:02.691191] hello_bdev.c: 308:main: *ERROR*: ERROR starting application

在网上找到了相应issue，https://github.com/spdk/spdk/issues/1550

正确的执行方式为：

./build/examples/hello_bdev -c ./examples/bdev/hello_world/bdev.json -b Malloc0


[2023-05-30 20:25:59.131197] Starting SPDK v23.05-pre git sha1 ee06693c3 / DPDK 22.11.1 initialization...
[2023-05-30 20:25:59.132037] [ DPDK EAL parameters: hello_bdev --no-shconf -c 0x1 --huge-unlink --log-level=lib.eal:6 --log-level=lib.cryptodev:5 --log-level=user1:6 --iova-mode=pa --base-virtaddr=0x200000000000 --match-allocations --file-prefix=spdk_pid11462 ]
TELEMETRY: No legacy callbacks, legacy socket not created
[2023-05-30 20:25:59.252268] app.c: 738:spdk_app_start: *NOTICE*: Total cores available: 1
[2023-05-30 20:25:59.303646] reactor.c: 937:reactor_run: *NOTICE*: Reactor started on core 0
[2023-05-30 20:25:59.359161] accel_sw.c: 601:sw_accel_module_init: *NOTICE*: Accel framework software module initialized.
[2023-05-30 20:25:59.387635] hello_bdev.c: 222:hello_start: *NOTICE*: Successfully started the application
[2023-05-30 20:25:59.388053] hello_bdev.c: 231:hello_start: *NOTICE*: Opening the bdev Malloc0
[2023-05-30 20:25:59.388153] hello_bdev.c: 244:hello_start: *NOTICE*: Opening io channel
[2023-05-30 20:25:59.388529] hello_bdev.c: 138:hello_write: *NOTICE*: Writing to the bdev
[2023-05-30 20:25:59.388757] hello_bdev.c: 117:write_complete: *NOTICE*: bdev io write completed successfully
[2023-05-30 20:25:59.388931] hello_bdev.c:  84:hello_read: *NOTICE*: Reading io
[2023-05-30 20:25:59.389019] hello_bdev.c:  65:read_complete: *NOTICE*: Read string from bdev : Hello World!

[2023-05-30 20:25:59.389128] hello_bdev.c:  74:read_complete: *NOTICE*: Stopping app

命令行参数

-b参数

static char *g_bdev_name = "Malloc0";
/*
 * Usage function for printing parameters that are specific to this application
 */
static void
hello_bdev_usage(void)
{
	printf(" -b <bdev>                 name of the bdev to use\n");
}

/*
 * This function is called to parse the parameters that are specific to this application
 */
static int
hello_bdev_parse_arg(int ch, char *arg)
{
	switch (ch) {
	case 'b':
		g_bdev_name = arg;
		break;
	default:
		return -EINVAL;
	}
	return 0;
}
spdk_app_parse_args(argc, argv, &opts, "b:", NULL, hello_bdev_parse_arg, hello_bdev_usage)
hello_context.bdev_name = g_bdev_name;

可以看出，g_bdev_name本来就是Malloc0，-b Malloc0没啥用

-c参数

static void
usage(void (*app_usage)(void))
{
	printf("%s [options]\n", g_executable_name);
	printf("options:\n");
	printf(" -c, --config <config>     JSON config file (default %s)\n",
	       g_default_opts.json_config_file != NULL ? g_default_opts.json_config_file : "none");

-c后加json配置文件名，bdev.json文件内容如下：

{
  "subsystems": [
    {
      "subsystem": "bdev",
      "config": [
        {
          "method": "bdev_malloc_create",
          "params": {
            "name": "Malloc0",
            "num_blocks": 32768,
            "block_size": 512
          }
        }
      ]
    }
  ]
}

简要看json的解析过程，全局查询json_config_file，找到spdk_subsystem_init_from_json_config函数

void
spdk_subsystem_init_from_json_config(const char *json_config_file, const char *rpc_addr,
				     spdk_subsystem_init_fn cb_fn, void *cb_arg,
				     bool stop_on_error)
{
	struct load_json_config_ctx *ctx = calloc(1, sizeof(*ctx));
	int rc;

	assert(cb_fn);
	if (!ctx) {
		cb_fn(-ENOMEM, cb_arg);
		return;
	}

	ctx->cb_fn = cb_fn;
	ctx->cb_arg = cb_arg;
	ctx->stop_on_error = stop_on_error;
	ctx->thread = spdk_get_thread();

	rc = app_json_config_read(json_config_file, ctx);
	if (rc) {
		goto fail;
	}

	/* Capture subsystems array */
	rc = spdk_json_find_array(ctx->values, "subsystems", NULL, &ctx->subsystems);
	switch (rc) {
	case 0:
		/* Get first subsystem */
		ctx->subsystems_it = spdk_json_array_first(ctx->subsystems);
		if (ctx->subsystems_it == NULL) {
			SPDK_NOTICELOG("'subsystems' configuration is empty\n");
		}
		break;
	case -EPROTOTYPE:
		SPDK_ERRLOG("Invalid JSON configuration: not enclosed in {}.\n");
		goto fail;
	case -ENOENT:
		SPDK_WARNLOG("No 'subsystems' key JSON configuration file.\n");
		break;
	case -EDOM:
		SPDK_ERRLOG("Invalid JSON configuration: 'subsystems' should be an array.\n");
		goto fail;
	default:
		SPDK_ERRLOG("Failed to parse JSON configuration.\n");
		goto fail;
	}

	/* If rpc_addr is not an Unix socket use default address as prefix. */
	if (rpc_addr == NULL || rpc_addr[0] != '/') {
		rpc_addr = SPDK_DEFAULT_RPC_ADDR;
	}

	/* FIXME: rpc client should use socketpair() instead of this temporary socket nonsense */
	rc = snprintf(ctx->rpc_socket_path_temp, sizeof(ctx->rpc_socket_path_temp), "%s.%d_config",
		      rpc_addr, getpid());
	if (rc >= (int)sizeof(ctx->rpc_socket_path_temp)) {
		SPDK_ERRLOG("Socket name create failed\n");
		goto fail;
	}

	rc = spdk_rpc_initialize(ctx->rpc_socket_path_temp);
	if (rc) {
		goto fail;
	}

	ctx->client_conn = spdk_jsonrpc_client_connect(ctx->rpc_socket_path_temp, AF_UNIX);
	if (ctx->client_conn == NULL) {
		SPDK_ERRLOG("Failed to connect to '%s'\n", ctx->rpc_socket_path_temp);
		goto fail;
	}

	rpc_client_set_timeout(ctx, RPC_CLIENT_CONNECT_TIMEOUT_US);
	ctx->client_conn_poller = SPDK_POLLER_REGISTER(rpc_client_connect_poller, ctx, 100);
	return;

fail:
	app_json_config_load_done(ctx, -EINVAL);
}

全局查询bdev_malloc_create，找到rpc_bdev_malloc_create函数

static void
rpc_bdev_malloc_create(struct spdk_jsonrpc_request *request,
		       const struct spdk_json_val *params)
{
	struct malloc_bdev_opts req = {NULL};
	struct spdk_json_write_ctx *w;
	struct spdk_bdev *bdev;
	int rc = 0;

	if (spdk_json_decode_object(params, rpc_construct_malloc_decoders,
				    SPDK_COUNTOF(rpc_construct_malloc_decoders),
				    &req)) {
		SPDK_DEBUGLOG(bdev_malloc, "spdk_json_decode_object failed\n");
		spdk_jsonrpc_send_error_response(request, SPDK_JSONRPC_ERROR_INTERNAL_ERROR,
						 "spdk_json_decode_object failed");
		goto cleanup;
	}

	rc = create_malloc_disk(&bdev, &req);
	if (rc) {
		spdk_jsonrpc_send_error_response(request, rc, spdk_strerror(-rc));
		goto cleanup;
	}

	free_rpc_construct_malloc(&req);

	w = spdk_jsonrpc_begin_result(request);
	spdk_json_write_string(w, spdk_bdev_get_name(bdev));
	spdk_jsonrpc_end_result(request, w);
	return;

cleanup:
	free_rpc_construct_malloc(&req);
}
SPDK_RPC_REGISTER("bdev_malloc_create", rpc_bdev_malloc_create, SPDK_RPC_RUNTIME)  

运行到该函数的回溯栈为

(gdb) bt
#0  rpc_bdev_malloc_create (request=0xcac1f474c379e400, params=0x555555cc9570) at bdev_malloc_rpc.c:49
#1  0x00005555556b0a53 in jsonrpc_handler (request=0x555555cc04e0, method=0x555555c648e0, params=0x555555c64900) at rpc.c:124
#2  0x00005555556b2c5e in jsonrpc_server_handle_request (request=0x555555cc04e0, method=0x555555c648e0, params=0x555555c64900) at jsonrpc_server_tcp.c:222
#3  0x00005555556b1665 in parse_single_request (request=0x555555cc04e0, values=0x555555c64880) at jsonrpc_server.c:75
#4  0x00005555556b1c68 in jsonrpc_parse_request (conn=0x7ffff5f7e040, json=0x7ffff5f7e058, size=172) at jsonrpc_server.c:205
#5  0x00005555556b2eaa in jsonrpc_server_conn_recv (conn=0x7ffff5f7e040) at jsonrpc_server_tcp.c:284
#6  0x00005555556b3297 in spdk_jsonrpc_server_poll (server=0x7ffff5f7e010) at jsonrpc_server_tcp.c:402
#7  0x00005555556b0d59 in spdk_rpc_accept () at rpc.c:213
#8  0x00005555556a13c4 in rpc_subsystem_poll (arg=0x0) at rpc.c:21
#9  0x00005555556a82fd in thread_execute_timed_poller (thread=0x555555c9ec00, poller=0x555555cbf2c0, now=41542509569737) at thread.c:970
#10 0x00005555556a8613 in thread_poll (thread=0x555555c9ec00, max_msgs=0, now=41542509569737) at thread.c:1060
#11 0x00005555556a8837 in spdk_thread_poll (thread=0x555555c9ec00, max_msgs=0, now=41542509569737) at thread.c:1119
#12 0x000055555566d309 in _reactor_run (reactor=0x555555c7b780) at reactor.c:914
#13 0x000055555566d3fb in reactor_run (arg=0x555555c7b780) at reactor.c:952
#14 0x000055555566d887 in spdk_reactors_start () at reactor.c:1068
#15 0x0000555555669c5d in spdk_app_start (opts_user=0x7fffffffdea0, start_fn=0x55555556e1fb <hello_start>, arg1=0x7fffffffde40) at app.c:779
#16 0x000055555556e5d9 in main (argc=5, argv=0x7fffffffe078) at hello_bdev.c:306

p req
$19 = {name = 0x555555cc9580 "Malloc0", uuid = {u = {raw = '\000' <repeats 15 times>}}, num_blocks = 32768, block_size = 512, physical_block_size = 0, optimal_io_boundary = 0, md_size = 0, 
  md_interleave = false, dif_type = SPDK_DIF_DISABLE, dif_is_head_of_md = false}

猜测rpc_bdev_malloc_create函数与spdk_subsystem_init_from_json_config中的
SPDK_POLLER_REGISTER(rpc_client_connect_poller, ctx, 100);有关。

有兴趣的可以继续研究rpc_bdev_malloc_create函数中的create_malloc_disk函数