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

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Message-ID: <78d13292-0cfe-31b6-7a9c-daf7fb7f3d23@arm.com>
Date:   Fri, 25 Oct 2019 15:40:36 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Christophe Leroy <christophe.leroy@....fr>, 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



On 10/25/2019 02:22 PM, Christophe Leroy wrote:
> 
> 
> 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 ?

IIRC, probably this was for later if and when the vaddr calculation becomes
dependent on other factors rather than this simple arithmetic involving start
and end of process address space on a platform.

> 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.

You are right.

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

I was mistaken. We dont need those checks for now, hence will drop them next time.

> 
> Christophe
>