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Subject: Re: [PATCH V7] mm/debug: Add tests validating architecture page table
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To:     Anshuman Khandual <anshuman.khandual@arm.com>,
        Qian Cai <cai@lca.pw>
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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <d811622e-0d35-3bc6-9568-36abc1bee355@c-s.fr>
Date:   Fri, 25 Oct 2019 10:52:54 +0200
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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@arm.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