linux-kernel - Re: [PATCH V7] mm/debug: Add tests validating architecture page table helpers

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Date:   Fri, 25 Oct 2019 10:52:54 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Qian Cai <cai@....pw>
Cc:     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>,
        Mike Kravetz <mike.kravetz@...cle.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
Subject: Re: [PATCH V7] mm/debug: Add tests validating architecture page table
 helpers



Le 25/10/2019 à 10:24, Anshuman Khandual a écrit :
> 
> 
> On 10/25/2019 12:41 PM, Christophe Leroy wrote:
>>
>>
>> Le 25/10/2019 à 07:52, Qian Cai a écrit :
>>>
>>>
>>>> On Oct 24, 2019, at 11:45 PM, Anshuman Khandual <Anshuman.Khandual@....com> wrote:
>>>>
>>>> Nothing specific. But just tested this with x86 defconfig with relevant configs
>>>> which are required for this test. Not sure if it involved W=1.
>>>
>>> No, it will not. It needs to run like,
>>>
>>> make W=1 -j 64 2>/tmp/warns
>>>
>>
>> Are we talking about this peace of code ?
>>
>> +static unsigned long __init get_random_vaddr(void)
>> +{
>> +    unsigned long random_vaddr, random_pages, total_user_pages;
>> +
>> +    total_user_pages = (TASK_SIZE - FIRST_USER_ADDRESS) / PAGE_SIZE;
>> +
>> +    random_pages = get_random_long() % total_user_pages;
>> +    random_vaddr = FIRST_USER_ADDRESS + random_pages * PAGE_SIZE;
>> +
>> +    WARN_ON((random_vaddr > TASK_SIZE) ||
>> +        (random_vaddr < FIRST_USER_ADDRESS));
>> +    return random_vaddr;
>> +}
>> +
>>
>> ramdom_vaddr is unsigned,
>> random_pages is unsigned and lower than total_user_pages
>>
>> So the max value random_vaddr can get is FIRST_USER_ADDRESS + ((TASK_SIZE - FIRST_USER_ADDRESS - 1) / PAGE_SIZE) * PAGE_SIZE = TASK_SIZE - 1
>> And the min value random_vaddr can get is FIRST_USER_ADDRESS (that's when random_pages = 0)
> 
> That's right.
> 
>>
>> So the WARN_ON() is just unneeded, isn't it ?
> 
> It is just a sanity check on possible vaddr values before it's corresponding
> page table mappings could be created. If it's worth to drop this in favor of
> avoiding these unwanted warning messages on x86, will go ahead with it as it
> is not super important.
> 

But you are checking what ? That the compiler does calculation correctly 
or what ?
As mentionned just above, based on the calculation done, what you are 
testing cannot happen, so I'm having a hard time understanding what kind 
of sanity check it can be.

Can you give an exemple of a situation which could trigger the warning ?

Christophe