AVR Freaks

I am working on ATSAMC21E17 mcu with ASF4 in atmel studio 7.

I write data to the embedded RWW section of internal flash using the following code, it works fine.

    uint8_t  config_data[64];
    uint32_t page_size;
    uint8_t i,j;

    /* Init source data */
    page_size = _rww_flash_get_page_size(&FLASH_0.dev);

    /*Write all the 4K RWWEE flash*/

    for (j=0;j<64;j++)
    {
        for (i = 0; i < page_size; i++) {
            config_data[i] = j;
        }

        /* Write data to RWWEE flash */
        if (_rww_flash_write(&FLASH_0.dev, NVMCTRL_RWW_EEPROM_ADDR, config_data, page_size) != ERR_NONE) {
            while (1)
            ; /* Trap here when flash write error happen */
        }
    }

I read the flash out into a file. I found the data.

But when I want to read back the data. I got all 0xFF. The code is :

    for (i = 0; i < 64; i++) {
        /* Read data from RWWEE flash */
        if (_rww_flash_read(&FLASH_0.dev, NVMCTRL_RWW_EEPROM_ADDR+i*page_size, config_data, page_size) != ERR_NONE) {
            while (1)
            ; /* Trap here when flash write error happen */
        }
    }

Most code is copy from the ASF4 example. So what's wrong? Can somebody tell me the reason?

Thanks !