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Subject: Re: [PATCH V7] mm/debug: Add tests validating architecture page table
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To:     Christophe Leroy <christophe.leroy@c-s.fr>, Qian Cai <cai@lca.pw>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
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From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <78d13292-0cfe-31b6-7a9c-daf7fb7f3d23@arm.com>
Date:   Fri, 25 Oct 2019 15:40:36 +0530
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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@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 ?

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
>