linux-kernel - Re: [PATCH v2 07/13] powerpc: add support for folded p4d page tables

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Date:   Wed, 26 Feb 2020 12:20:49 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     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>,
        Russell King <linux@...linux.org.uk>,
        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 07/13] powerpc: add support for folded p4d page tables



Le 26/02/2020 à 11:56, Mike Rapoport a écrit :
> On Wed, Feb 26, 2020 at 10:46:13AM +0100, Christophe Leroy wrote:
>>
>>
>> Le 26/02/2020 à 10:13, Mike Rapoport a écrit :
>>> On Tue, Feb 18, 2020 at 12:54:40PM +0200, Mike Rapoport wrote:
>>>> On Sun, Feb 16, 2020 at 11:41:07AM +0100, Christophe Leroy wrote:
>>>>>
>>>>>
>>>>> Le 16/02/2020 à 09:18, Mike Rapoport a écrit :
>>>>>> From: Mike Rapoport <rppt@...ux.ibm.com>
>>>>>>
>>>>>> Implement primitives necessary for the 4th level folding, add walks of p4d
>>>>>> level where appropriate and replace 5level-fixup.h with pgtable-nop4d.h.
>>>>>
>>>>> I don't think it is worth adding all this additionnals walks of p4d, this
>>>>> patch could be limited to changes like:
>>>>>
>>>>> -		pud = pud_offset(pgd, gpa);
>>>>> +		pud = pud_offset(p4d_offset(pgd, gpa), gpa);
>>>>>
>>>>> The additionnal walks should be added through another patch the day powerpc
>>>>> need them.
>>>>
>>>> Ok, I'll update the patch to reduce walking the p4d.
>>>
>>> Here's what I have with more direct acceses from pgd to pud.
>>
>> I went quickly through. This looks promising.
>>
>> Do we need the walk_p4d() in arch/powerpc/mm/ptdump/hashpagetable.c ?
>> Can't we just do
>>
>> @@ -445,7 +459,7 @@ static void walk_pagetables(struct pg_state *st)
>>   		addr = KERN_VIRT_START + i * PGDIR_SIZE;
>>   		if (!pgd_none(*pgd))
>>   			/* pgd exists */
>> -			walk_pud(st, pgd, addr);
>> +			walk_pud(st, p4d_offset(pgd, addr), addr);
> 
> We can do
> 
> 	addr = KERN_VIRT_START + i * PGDIR_SIZE;
> 	p4d = p4d_offset(pgd, addr);
> 	if (!p4d_none(*pgd))
> 		walk_pud()
> 
> But I don't think this is really essential. Again, we are trading off code
> consistency vs line count. I don't think line count is that important.

Ok.

Christophe