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

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Message-Id: <50B7EFFE-1FF0-4B16-84A1-D851052B18B4@lca.pw>
Date:   Wed, 29 Jan 2020 06:09:56 -0500
From:   Qian Cai <cai@....pw>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     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>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux-MM <linux-mm@...ck.org>, Paul Mackerras <paulus@...ba.org>,
        sparclinux@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        linux-s390@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>,
        Michael Ellerman <mpe@...erman.id.au>,
        Vlastimil Babka <vbabka@...e.cz>,
        the arch/x86 maintainers <x86@...nel.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Steven Price <Steven.Price@....com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-arm-kernel@...ts.infradead.org,
        linux-snps-arc@...ts.infradead.org,
        Kees Cook <keescook@...omium.org>,
        Anshuman Khandual <Anshuman.Khandual@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Mark Brown <broonie@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Thomas Gleixner <tglx@...utronix.de>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Sri Krishna chowdary <schowdary@...dia.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.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 Jan 29, 2020, at 5:36 AM, Catalin Marinas <catalin.marinas@....com> wrote:
> 
> On Tue, Jan 28, 2020 at 02:07:10PM -0500, Qian Cai wrote:
>> On Jan 28, 2020, at 12:47 PM, Catalin Marinas <catalin.marinas@....com> wrote:
>>> 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.
>> 
>> This still does not answer the fundamental question. If this test is
>> simply inefficient to find bugs,
> 
> Who said this is inefficient (other than you)?

Inefficient of finding bugs. It said only found a bug or two in its lifetime?

> 
>> who wants to spend time to use it regularly? 
> 
> Arch maintainers, mm maintainers introducing new macros or assuming
> certain new semantics of the existing macros.
> 
>> If this is just one off test that may get running once in a few years
>> (when introducing a new arch), how does it justify the ongoing cost to
>> maintain it?
> 
> You are really missing the point. It's not only for a new arch but
> changes to existing arch code. And if the arch code churn in this area
> is relatively small, I'd expect a similarly small cost of maintaining
> this test.
> 
> If you only turn DEBUG_VM on once every few years, don't generalise this
> to the rest of the kernel developers (as others pointed out, this test
> is default y if DEBUG_VM).

Quite the opposite, I am running DEBUG_VM almost daily for regression
workload while I felt strongly this thing does not add any value mixing there.

So, I would suggest to decouple this away from DEBUG_VM, and clearly
document that this test is not something intended for automated regression
workloads, so those people don’t need to waste time running this.

> 
> Anyway, I think that's a pointless discussion, so not going to reply
> further (unless you have technical content to add).
> 
> -- 
> Catalin