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

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Date:   Mon, 03 Feb 2020 10:48:14 -0500
From:   Qian Cai <cai@....pw>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Anshuman Khandual <Anshuman.Khandual@....com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mark Rutland <Mark.Rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Steven Price <Steven.Price@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Matthew Wilcox <willy@...radead.org>,
        Sri Krishna chowdary <schowdary@...dia.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Vineet Gupta <vgupta@...opsys.com>,
        James Hogan <jhogan@...nel.org>,
        Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-snps-arc@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH V12] mm/debug: Add tests validating architecture page
 table helpers

On Mon, 2020-02-03 at 16:14 +0100, Christophe Leroy wrote:
> 
> Le 02/02/2020 à 12:26, Qian Cai a écrit :
> > 
> > 
> > > On Jan 30, 2020, at 9:13 AM, Christophe Leroy <christophe.leroy@....fr> wrote:
> > > 
> > > config DEBUG_VM_PGTABLE
> > >     bool "Debug arch page table for semantics compliance" if ARCH_HAS_DEBUG_VM_PGTABLE || EXPERT
> > >     depends on MMU
> > >     default 'n' if !ARCH_HAS_DEBUG_VM_PGTABLE
> > >     default 'y' if DEBUG_VM
> > 
> > Does it really necessary to potentially force all bots to run this? Syzbot, kernel test robot etc? Does it ever pay off for all their machine times there?
> > 
> 
> Machine time ?
> 
> On a 32 bits powerpc running at 132 MHz, the tests takes less than 10ms. 
> Is it worth taking the risk of not detecting faults by not selecting it 
> by default ?

The risk is quite low as Catalin mentioned this thing is not to detect
regressions but rather for arch/mm maintainers.

I do appreciate the efforts to get everyone as possible to run this thing,
so it get more notices once it is broken. However, DEBUG_VM seems like such
a generic Kconfig those days that have even been enabled by default for
Fedora Linux, so I would rather see a more sensitive default been taken
even though the test runtime is fairly quickly on a small machine for now.

> 
> [    5.656916] debug_vm_pgtable: debug_vm_pgtable: Validating 
> architecture page table helpers
> [    5.665661] debug_vm_pgtable: debug_vm_pgtable: Validated 
> architecture page table helpers