linux-kernel - Re: question on lazy tlb flush

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Date:   Wed, 29 May 2019 10:17:01 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: question on lazy tlb flush

On Wed, 2019-05-29 at 12:54 +0800, Zhenzhong Duan wrote:
> 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?

That is correct, and you ask a very good question.

The software does indeed not access userspace memory
addresses from kernel threads.

However, the CPU itself can do speculative loads from
userspace memory addresses, even when the software thread
is running exclusively in kernel mode.

Add to that the fact that the page table pages can get
reused for something else after they have been freed, and
that the CPU can cache intermediate translations (eg. PUD
and PMD level things get cached in the TLB), and you might
end up with a speculative load poking at a PCI register,
or something else that trips up the machine.

For that reason, when page table pages get freed, we need
to flush the TLBs of all users, inluding lazy TLB kernel
threads.

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