linux-kernel - Re: [RFC 01/20] mm/tlb: fix fullmm semantics

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Date:   Wed, 3 Feb 2021 19:20:26 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Will Deacon <will@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu Zhao <yuzhao@...gle.com>, Nick Piggin <npiggin@...il.com>,
        X86 ML <x86@...nel.org>, Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 01/20] mm/tlb: fix fullmm semantics

> On Feb 3, 2021, at 1:44 AM, Will Deacon <will@...nel.org> wrote:
> 
> On Tue, Feb 02, 2021 at 01:35:38PM -0800, Nadav Amit wrote:
>>> On Feb 2, 2021, at 3:00 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> 
>>> On Tue, Feb 02, 2021 at 01:32:36AM -0800, Nadav Amit wrote:
>>>>> On Feb 1, 2021, at 3:36 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>>>> 
>>>>> 
>>>>> https://lkml.kernel.org/r/20210127235347.1402-1-will@kernel.org
>>>> 
>>>> I have seen this series, and applied my patches on it.
>>>> 
>>>> Despite Will’s patches, there were still inconsistencies between fullmm
>>>> and need_flush_all.
>>>> 
>>>> Am I missing something?
>>> 
>>> I wasn't aware you were on top. I'll look again.
>> 
>> Looking on arm64’s tlb_flush() makes me think that there is currently a bug
>> that this patch fixes. Arm64’s tlb_flush() does:
>> 
>>       /*
>>        * If we're tearing down the address space then we only care about
>>        * invalidating the walk-cache, since the ASID allocator won't
>>        * reallocate our ASID without invalidating the entire TLB.
>>        */
>>       if (tlb->fullmm) {
>>               if (!last_level)
>>                       flush_tlb_mm(tlb->mm);
>>               return;
>>       } 
>> 
>> But currently tlb_mmu_finish() can mistakenly set fullmm incorrectly (if
>> mm_tlb_flush_nested() is true), which might skip the TLB flush.
> 
> But in that case isn't 'freed_tables' set to 1, so 'last_level' will be
> false and we'll do the flush in the code above?

Indeed. You are right. So no rush.