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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>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        Mark Brown <broonie@kernel.org>,
        Steven Price <Steven.Price@arm.com>,
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        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Kees Cook <keescook@chromium.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
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 <3cfec421-4006-4159-ca32-313ff5196ff9@c-s.fr>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <763d58b4-f532-0bba-bf2b-71433ac514fb@arm.com>
Date:   Fri, 25 Oct 2019 13:54:14 +0530
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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.

> 
> Christophe
>