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

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Message-Id: <9E576E79-E4FD-4CB9-8BE5-142230C12E63@gmail.com>
Date:   Fri, 22 Oct 2021 14:58:39 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        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 Oct 21, 2021, at 8:04 PM, Andrew Morton <akpm@...ux-foundation.org> 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.


There is no functionality bug in the kernel. The Knights Landing bug
was circumvented eventually by changing the swap entry structure so
the access/dirty bits would not overlap with the swap entry data.

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

I included some data before [1]. In general the cost that is saved
is the cost of a TLB flush/shootdown.

I will modify my benchmark to test huge-pages (which were not
included in the previous patch-set) and send results later. I would
also try nodejs to see if there is a significant enough benefit.
Nodejs crashed before (hence the 3rd patch added here), as it
exec-protects/unprotects pages - I will see if the benefit shows in
the benchmarks.

[ The motivation behind the patches is to later introduce userfaultfd
writeprotectv interface, and for my use-case that is under 
development this proved to improve performance considerably. ]



[1] https://lore.kernel.org/linux-mm/DA49DBBB-FFEE-4ACC-BB6C-364D07533C5E@vmware.com/