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

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Date:   Tue, 28 Jan 2020 17:47:09 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Qian Cai <cai@....pw>
Cc:     Anshuman Khandual <Anshuman.Khandual@....com>,
        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>, linux-mm@...ck.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>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        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 Mon, Jan 27, 2020 at 09:11:53PM -0500, Qian Cai wrote:
> On Jan 27, 2020, at 8:28 PM, Anshuman Khandual <Anshuman.Khandual@....com> wrote:
> > This adds tests which will validate architecture page table helpers and
> > other accessors in their compliance with expected generic MM semantics.
> > This will help various architectures in validating changes to existing
> > page table helpers or addition of new ones.
[...]
> What’s the value of this block of new code? It only supports x86 and
> arm64 which are supposed to be good now. Did those tests ever find any
> regression or this is almost only useful for new architectures which
> only happened once in a few years?

The primary goal here is not finding regressions but having clearly
defined semantics of the page table accessors across architectures. x86
and arm64 are a good starting point and other architectures will be
enabled as they are aligned to the same semantics.

See for example this past discussion:

https://lore.kernel.org/linux-mm/20190628102003.GA56463@arrakis.emea.arm.com/

These tests should act as the 'contract' between the generic mm code and
the architecture port. Without clear semantics, some bugs may be a lot
subtler than a boot failure.

FTR, I fully support this patch (and I should get around to review it
properly; thanks for the reminder ;)).

-- 
Catalin