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

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Message-ID: <YXaL2Cq7XI/gQNCZ@hirez.programming.kicks-ass.net>
Date:   Mon, 25 Oct 2021 12:50:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nadav Amit <nadav.amit@...il.com>, 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>,
        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, Oct 21, 2021 at 08:04:50PM -0700, Andrew Morton wrote:
> 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.

I am confused, 1/5 doesn't actually do *anything*. I also cannot find
any further usage of the introduced X86_BUG_PTE_LEAK.

I'm thinking patch #2 means to have something like:

	if (cpu_feature_enabled(X86_BUG_PTE_LEAK))
		flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);

In the newly minted: pmdp_invalidate_ad(), but alas, nothing there.