linux-kernel - Re: [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes

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Message-Id: <20211021200450.b13499c379a27dbfefe9f5e3@linux-foundation.org>
Date:   Thu, 21 Oct 2021 20:04:50 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Nadav Amit <namit@...are.com>, Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>, x86@...nel.org
Subject: Re: [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes

On Thu, 21 Oct 2021 05:21:07 -0700 Nadav Amit <nadav.amit@...il.com> wrote:

> This patch-set is intended to remove unnecessary TLB flushes. It is
> based on feedback from v1 and several bugs I found in v1 myself.
> 
> Basically, there are 3 optimizations in this patch-set:
> 1. Avoiding TLB flushes on change_huge_pmd() that are only needed to
>    prevent the A/D bits from changing.
> 2. Use TLB batching infrastructure to batch flushes across VMAs and
>    do better/fewer flushes.
> 3. Avoid TLB flushes on permission demotion.
> 
> Andrea asked for the aforementioned (2) to come after (3), but this
> is not simple (specifically since change_prot_numa() needs the number
> of pages affected).

[1/5] appears to be a significant fix which should probably be
backported into -stable kernels.  If you agree with this then I suggest
it be prepared as a standalone patch, separate from the other four
patches.  With a cc:stable.

And the remaining patches are a performance optimization.  Has any
attempt been made to quantify the benefits?