linux-kernel - Re: [RFC 11/20] mm/tlb: remove arch-specific tlb_start/end

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Date:   Tue, 2 Feb 2021 07:20:55 +0000
From:   Nadav Amit <namit@...are.com>
To:     Nicholas Piggin <npiggin@...il.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, X86 ML <x86@...nel.org>,
        Yu Zhao <yuzhao@...gle.com>
Subject: Re: [RFC 11/20] mm/tlb: remove arch-specific tlb_start/end_vma()

> On Feb 1, 2021, at 10:41 PM, Nicholas Piggin <npiggin@...il.com> wrote:
> 
> Excerpts from Peter Zijlstra's message of February 1, 2021 10:09 pm:
>> I also don't think AGRESSIVE_FLUSH_BATCHING quite captures what it does.
>> How about:
>> 
>> 	CONFIG_MMU_GATHER_NO_PER_VMA_FLUSH
> 
> Yes please, have to have descriptive names.

Point taken. I will fix it.

> 
> I didn't quite see why this was much of an improvement though. Maybe 
> follow up patches take advantage of it? I didn't see how they all fit 
> together.

They do, but I realized as I said in other emails that I have a serious bug
in the deferred invalidation scheme.

Having said that, I think there is an advantage of having an explicit config
option instead of relying on whether tlb_end_vma is defined. For instance,
Arm does not define tlb_end_vma, and consequently it flushes the TLB after
each VMA. I suspect it is not intentional.