Some places in the kernel can not afford to have memory allocations fail. For example, memory pressure can force the system to swap pages out, but that swap operation will require memory to be executed. If the memory to set up the swap is not available, the system comes to a halt.
负担不起的内存短缺。例如，为了缓过来解内存不足的情况，需要交换页出内存，但是交换操作本身也需内存来完成操作，这样就可能掉坑里。
System administrators, when faced with that prospect, tend to break out in a cold sweat, overindulge in beer, and switch to BSD.
系统管理员看到这副场景，肯定吓出一身冷汗，举杯庆祝，转向BSD
https://lwn.net/2001/1213/kernel.php3
