linux-kernel - Re: [RFC PATCH v5 4/8] mm: tlb: Pass struct mmu_gather to flush_pmd_tlb

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Date:   Fri, 3 Apr 2020 13:14:21 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <mark.rutland@....com>, <will@...nel.org>,
        <catalin.marinas@....com>, <aneesh.kumar@...ux.ibm.com>,
        <akpm@...ux-foundation.org>, <npiggin@...il.com>, <arnd@...db.de>,
        <rostedt@...dmis.org>, <maz@...nel.org>, <suzuki.poulose@....com>,
        <tglx@...utronix.de>, <yuzhao@...gle.com>, <Dave.Martin@....com>,
        <steven.price@....com>, <broonie@...nel.org>,
        <guohanjun@...wei.com>, <corbet@....net>, <vgupta@...opsys.com>,
        <tony.luck@...el.com>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-mm@...ck.org>, <arm@...nel.org>, <xiexiangyou@...wei.com>,
        <prime.zeng@...ilicon.com>, <zhangshaokun@...ilicon.com>,
        <kuhn.chenqun@...wei.com>
Subject: Re: [RFC PATCH v5 4/8] mm: tlb: Pass struct mmu_gather to
 flush_pmd_tlb_range

Hi Peter,

On 2020/4/3 0:38, Peter Zijlstra wrote:
> On Thu, Apr 02, 2020 at 07:24:04PM +0800, Zhenyu Ye wrote:
>> Thanks for your detailed explanation.  I notice that you used
>> `tlb_end_vma` replace `flush_tlb_range`, which will call `tlb_flush`,
>> then finally call `flush_tlb_range` in generic code.  However, some
>> architectures define tlb_end_vma|tlb_flush|flush_tlb_range themselves,
>> so this may cause problems.
>>
>> For example, in s390, it defines:
>>
>> #define tlb_end_vma(tlb, vma)			do { } while (0)
>>
>> And it doesn't define it's own flush_pmd_tlb_range().  So there will be
>> a mistake if we changed flush_pmd_tlb_range() using tlb_end_vma().
>>
>> Is this really a problem or something I understand wrong ?
> 
> If tlb_end_vma() is a no-op, then tlb_finish_mmu() will do:
> tlb_flush_mmu() -> tlb_flush_mmu_tlbonly() -> tlb_flush()
> 
> And s390 has tlb_flush().
> 
> If tlb_end_vma() is not a no-op and it calls tlb_flush_mmu_tlbonly(),
> then tlb_finish_mmu()'s invocation of tlb_flush_mmu_tlbonly() will
> terniate early due o no flags set.
> 
> IOW, it should all just work.
> 
> 
> FYI the whole tlb_{start,end}_vma() thing is a only needed when the
> architecture doesn't implement tlb_flush() and instead default to using
> flush_tlb_range(), at which point we need to provide a 'fake' vma.
> 
> At the time I audited all architectures and they only look at VM_EXEC
> (to do $I invalidation) and VM_HUGETLB (for pmd level invalidations),
> but I forgot which architectures that were.

Many architectures, such as alpha, arc, arm and so on.
I really understand why you hate making vma->vm_flags more important for
tlbi :).

> But that is all legacy code; eventually we'll get all archs a native
> tlb_flush() and this can go away.
> 

Thanks for your reply.  Currently, to enable the TTL feature, extending
the flush_*tlb_range() may be more convenient.
I will send a formal PATCH soon.

Thanks,
Zhenyu