linux-kernel - Re: [PATCH v2 00/13] mm: remove __ARCH_HAS_5LEVEL

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Date:   Sun, 16 Feb 2020 11:45:59 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Mike Rapoport <rppt@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Brian Cain <bcain@...eaurora.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Guan Xuetao <gxt@....edu.cn>,
        James Morse <james.morse@....com>,
        Jonas Bonn <jonas@...thpole.se>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        Marc Zyngier <maz@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Rich Felker <dalias@...c.org>,
        Stafford Horne <shorne@...il.com>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Tony Luck <tony.luck@...el.com>, Will Deacon <will@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        kvmarm@...ts.cs.columbia.edu, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-sh@...r.kernel.org, nios2-dev@...ts.rocketboards.org,
        openrisc@...ts.librecores.org,
        uclinux-h8-devel@...ts.sourceforge.jp,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v2 00/13] mm: remove __ARCH_HAS_5LEVEL_HACK



Le 16/02/2020 à 09:22, Russell King - ARM Linux admin a écrit :
> On Sun, Feb 16, 2020 at 10:18:30AM +0200, Mike Rapoport wrote:
>> From: Mike Rapoport <rppt@...ux.ibm.com>
>>
>> Hi,
>>
>> These patches convert several architectures to use page table folding and
>> remove __ARCH_HAS_5LEVEL_HACK along with include/asm-generic/5level-fixup.h.
>>
>> The changes are mostly about mechanical replacement of pgd accessors with p4d
>> ones and the addition of higher levels to page table traversals.
>>
>> All the patches were sent separately to the respective arch lists and
>> maintainers hence the "v2" prefix.
> 
> You fail to explain why this change which adds 488 additional lines of
> code is desirable.
> 

The purpose of the series, ie droping a HACK, is worth it.

However looking at the powerpc patch I have the feeling that this series 
goes behind its purpose.

The number additional lines could be deeply reduced I think if we limit 
the patches to the strict minimum, ie just do things like below instead 
of adding lots of handling of useless levels.

Instead of doing things like:

-	pud = NULL;
+	p4d = NULL;
  	if (pgd_present(*pgd))
-		pud = pud_offset(pgd, gpa);
+		p4d = p4d_offset(pgd, gpa);
+	else
+		new_p4d = p4d_alloc_one(kvm->mm, gpa);
+
+	pud = NULL;
+	if (p4d_present(*p4d))
+		pud = pud_offset(p4d, gpa);
  	else
  		new_pud = pud_alloc_one(kvm->mm, gpa);

It could be limited to:

  	if (pgd_present(*pgd))
-		pud = pud_offset(pgd, gpa);
+		pud = pud_offset(p4d_offset(pgd, gpa), gpa);
  	else
  		new_pud = pud_alloc_one(kvm->mm, gpa);


Christophe