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

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Message-ID: <20200402163849.GM20713@hirez.programming.kicks-ass.net>
Date:   Thu, 2 Apr 2020 18:38:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhenyu Ye <yezhenyu2@...wei.com>
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

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.

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