linux-kernel - question on lazy tlb flush

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Message-ID: <cd421c2c-8507-6652-2ef7-a6f3b20efcd2@oracle.com>
Date:   Wed, 29 May 2019 12:54:57 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     riel@...riel.com
Subject: question on lazy tlb flush

Hi Maintainers,

A question raised when I learned below code.  Appreciate any help me 
understand the code.

void native_flush_tlb_others(const struct cpumask *cpumask,
                              const struct flush_tlb_info *info)

{

...

         /*
          * If no page tables were freed, we can skip sending IPIs to
          * CPUs in lazy TLB mode. They will flush the CPU themselves
          * at the next context switch.
          *
          * However, if page tables are getting freed, we need to send the
          * IPI everywhere, to prevent CPUs in lazy TLB mode from tripping
          * up on the new contents of what used to be page tables, while
          * doing a speculative memory access.
          */
         if (info->freed_tables)
                 smp_call_function_many(cpumask, flush_tlb_func_remote,
                                (void *)info, 1);
         else
                 on_each_cpu_cond_mask(tlb_is_not_lazy, 
flush_tlb_func_remote,
                                 (void *)info, 1, GFP_ATOMIC, cpumask);

}

I just didn't understand how a kernel thread could trip up on the new 
contents of what used to be page tables. I presume the freed page tables 
are user mapping?

But kernel thread only access kernel address space, is kernel space also 
freed?

thanks

Zhenzhong