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Date: Wed, 27 Jun 2012 11:01:31 +0800
From: Gavin Shan <shangw@linux.vnet.ibm.com>
Subject: Re: [PATCH 1/5] mm/sparse: check size of struct mm_section
Message-ID: <20120627030130.GA14213@shangw>
Reply-To: Gavin Shan <shangw@linux.vnet.ibm.com>
References: <1340466776-4976-1-git-send-email-shangw@linux.vnet.ibm.com>
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To: Michal Hocko <mhocko@suse.cz>
Cc: Gavin Shan <shangw@linux.vnet.ibm.com>, linux-mm@kvack.org, rientjes@google.com, hannes@cmpxchg.org, akpm@linux-foundation.org

>> >> >
>> >> 
>> >> I should have clarified that more clear :-)
>> >> 
>> >> >> >
>> >> >> >Why? The memory is continuous, right?
>> >> >> 
>> >> >> Yes, the memory is conginous and the capacity of specific entry
>> >> >> in mem_section[NR_SECTION_ROOTS] has been defined as follows:
>> >> >> 
>> >> >> 
>> >> >> #define SECTIONS_PER_ROOT       (PAGE_SIZE / sizeof (struct mem_section))
>> >> >> 
>> >> >> Also, the memory is prone to be allocated from bootmem by function
>> >> >> alloc_bootmem_node(), which has PAGE_SIZE alignment. So I think it's
>> >> >> reasonable to introduce the extra check here from my personal view :-)
>> >> >
>> >> >No it is not necessary because we will never cross the page boundary
>> >> >because (SECTIONS_PER_ROOT uses an int division)
>> >> 
>> >> Current situation is that we don't cross the page foundary, but somebody
>> >> else might change the data struct (struct mem_section) in future. 
>> >
>> >No, this is safe even if the structure size changes (unless it is bigger
>> >than PAGE_SIZE).
>> 
>> Yeah, but it can't fully utilize the allocated memory chunk if the size of
>> the struct isn't aligned well.
>
>And you think that this is justification is sufficient to fail
>compilation? I don't think so...
>

Yeah, it's not reasonable to breat the compilation. So I'm not sure if
linux already had one macro to do warning for the case?

Thanks,
Gavin

>> 
>> Let me drop it in next revision :-)
>> 
>> Thanks,
>> Gavin
>
>-- 
>Michal Hocko
>SUSE Labs
>SUSE LINUX s.r.o.
>Lihovarska 1060/12
>190 00 Praha 9    
>Czech Republic
>

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