linux-kernel - Re: [RFC PATCH 0/9] Enable THP migration for all possible architectures

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Message-Id: <20180427193619.435eb53a@thinkpad>
Date:   Fri, 27 Apr 2018 19:36:19 +0200
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Zi Yan <zi.yan@...t.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Zi Yan <zi.yan@...rutgers.edu>,
        Vineet Gupta <vgupta@...opsys.com>,
        linux-snps-arc@...ts.infradead.org,
        Russell King <linux@...linux.org.uk>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Steve Capper <steve.capper@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        x86@...nel.org, Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Michal Hocko <mhocko@...e.com>, linux-mips@...ux-mips.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Ram Pai <linuxram@...ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        linuxppc-dev@...ts.ozlabs.org,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Janosch Frank <frankja@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, "Huang, Ying" <ying.huang@...el.com>
Subject: Re: [RFC PATCH 0/9] Enable THP migration for all possible
 architectures

On Thu, 26 Apr 2018 10:27:55 -0400
Zi Yan <zi.yan@...t.com> wrote:

> From: Zi Yan <zi.yan@...rutgers.edu>
> 
> Hi all,
> 
> THP migration is only enabled on x86_64 with a special
> ARCH_ENABLE_THP_MIGRATION macro. This patchset enables THP migration for
> all architectures that uses transparent hugepage, so that special macro can
> be dropped. Instead, THP migration is enabled/disabled via
> /sys/kernel/mm/transparent_hugepage/enable_thp_migration.
> 
> I grepped for TRANSPARENT_HUGEPAGE in arch folder and got 9 architectures that
> are supporting transparent hugepage. I mechanically add __pmd_to_swp_entry() and
> __swp_entry_to_pmd() based on existing __pte_to_swp_entry() and
> __swp_entry_to_pte() for all these architectures, except tile which is going to
> be dropped.

This will not work on s390, the pmd layout is very different from the pte
layout. Using __swp_entry/type/offset() on a pmd will go horribly wrong.
I currently don't see a chance to make this work for us, so please make/keep
this configurable, and do not configure it for s390.

Regards,
Gerald