我做了一些挖掘，发现内核的自动清理已经开始，/etc/kernel/postinst.d/apt-auto-removal并且文件指出：
# In the common case this results in two kernels saved (booted into the
# second-latest kernel, we install the latest kernel in an upgrade), but
# can save up to four. Kernel refers here to a distinct release, which can
# potentially be installed in multiple flavours counting as one kernel.
但该脚本远在我的脑海中，除了最后 4 个内核之外，我无法轻易找到如何自动清理任何内容。