linux-kernel - Re: [PATCH V12] mm/debug: Add tests validating architecture page table helpers

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Message-Id: <20200212185548.3274ec2e@thinkpad>
Date:   Wed, 12 Feb 2020 18:55:48 +0100
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>, linux-mm@...ck.org,
        Mark Rutland <mark.rutland@....com>,
        linux-ia64@...r.kernel.org, linux-sh@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        James Hogan <jhogan@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Paul Mackerras <paulus@...ba.org>, sparclinux@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-s390@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
        x86@...nel.org, Russell King - ARM Linux <linux@...linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Steven Price <Steven.Price@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Mark Brown <broonie@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Dan Williams <dan.j.williams@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christophe Leroy <christophe.leroy@....fr>,
        Sri Krishna chowdary <schowdary@...dia.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mips@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
        linux-kernel@...r.kernel.org, Paul Burton <paul.burton@...s.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH V12] mm/debug: Add tests validating architecture page
 table helpers

On Wed, 12 Feb 2020 15:12:54 +0530
Anshuman Khandual <anshuman.khandual@....com> wrote:

> >> +/*
> >> + * On s390 platform, the lower 12 bits are used to identify given page table
> >> + * entry type and for other arch specific requirements. But these bits might
> >> + * affect the ability to clear entries with pxx_clear(). So while loading up
> >> + * the entries skip all lower 12 bits in order to accommodate s390 platform.
> >> + * It does not have affect any other platform.
> >> + */
> >> +#define RANDOM_ORVALUE	(0xfffffffffffff000UL)  
> > 
> > I'd suggest you generate this mask with something like
> > GENMASK(BITS_PER_LONG, PAGE_SHIFT).  
> 
> IIRC the lower 12 bits constrains on s390 platform might not be really related
> to it's PAGE_SHIFT which can be a variable, but instead just a constant number.
> But can definitely use GENMASK or it's variants here.
> 
> https://lkml.org/lkml/2019/9/5/862

PAGE_SHIFT would be fine, it is 12 on s390. However, in order to be
more precise, we do not really need all 12 bits, only the last 4 bits.
So, something like this would work:

#define RANDOM_ORVALUE GENMASK(BITS_PER_LONG - 1, 4)

The text in the comment could then also be changed from 12 to 4, and
be a bit more specific on the fact that the impact on pxx_clear()
results from the dynamic page table folding logic on s390:

/*
 * On s390 platform, the lower 4 bits are used to identify given page table
 * entry type. But these bits might affect the ability to clear entries with
 * pxx_clear() because of how dynamic page table folding works on s390. So
 * while loading up the entries do not change the lower 4 bits.
 * It does not have affect any other platform.
 */