linux-kernel - Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()

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Date:   Mon, 4 Dec 2023 12:01:25 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Ryan Roberts <ryan.roberts@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        David Hildenbrand <david@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()



Le 15/11/2023 à 22:37, Andrew Morton a écrit :
> On Wed, 15 Nov 2023 16:30:05 +0000 Ryan Roberts <ryan.roberts@....com> wrote:
> 
>> However, the primary motivation for this change is to reduce the number
>> of tlb maintenance operations that the arm64 backend has to perform
>> during fork
> 
> Do you have a feeling for how much performance improved due to this?
> 
> Are there other architectures which might similarly benefit?  By
> implementing ptep_set_wrprotects(), it appears.  If so, what sort of
> gains might they see?
> 

I think powerpc 8xx would benefit.

We have 16k hugepages which are implemented with 4 identical PTE which 
have the _PAGE_SPS bit set. _PAGE_SPS says it is a 16k page.

We have 512k hugepages which are implemented with 128 identical PTE 
which have _PAGE_HUGE bit set. _PAGE_HUGE tells it is a 512k page.

FWIW, PMD size is 4M and there is no 4M page size so no way to implement 
leaf PMD huge pages.

Christophe