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Re: [PATCH V2 1/2] mm/memblock.c: trivial code refine in memblock_is_region_memory()

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On Mon, Dec 19, 2016 at 04:15:14PM +0100, Michal Hocko wrote:
>On Sun 18-12-16 14:47:49, Wei Yang wrote:
>> The base address is already guaranteed to be in the region by
>> memblock_search().
>

Hi, Michal

Nice to receive your comment.

>First of all the way how the check is removed is the worst possible...
>Apart from that it is really not clear to me why checking the base
>is not needed. You are mentioning memblock_search but what about other
>callers? adjust_range_page_size_mask e.g...
>

Hmm... the memblock_search() is called by memblock_is_region_memory(). Maybe I
paste the whole function here would clarify the change.

int __init_memblock memblock_is_region_memory(phys_addr_t base, phys_addr_t size)
{
	int idx = memblock_search(&memblock.memory, base);
	phys_addr_t end = base + memblock_cap_size(base, &size);

	if (idx == -1)
		return 0;
	return memblock.memory.regions[idx].base <= base &&
		(memblock.memory.regions[idx].base +
		 memblock.memory.regions[idx].size) >= end;
}

So memblock_search() will search "base" in memblock.memory. If "base" is not
in memblock.memory, idx would be -1. Then following code will not be executed.

And if the following code is executed, it means idx is not -1 and
memblock_search() has found the "base" in memblock.memory.regions[idx], which
is ture for statement (memblock.memory.regions[idx].base <= base).

>You also didn't mention what is the motivation of this change? What will
>work better or why it makes sense in general?
>

The purpose is to improve the code by reduce an extra check.

>Also this seems to be a general purpose function so it should better
>be robust.
>

I think it is as robust as it was.

>> This patch removes the check on base.
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>
>Without a proper justification and with the horrible way how it is done
>Nacked-by: Michal Hocko <mhocko@suse.com>
>

Not sure I make it clear or I may miss something?

>> ---
>>  mm/memblock.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index 7608bc3..cd85303 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -1615,7 +1615,7 @@ int __init_memblock memblock_is_region_memory(phys_addr_t base, phys_addr_t size
>>  
>>  	if (idx == -1)
>>  		return 0;
>> -	return memblock.memory.regions[idx].base <= base &&
>> +	return /* memblock.memory.regions[idx].base <= base && */
>>  		(memblock.memory.regions[idx].base +
>>  		 memblock.memory.regions[idx].size) >= end;
>>  }
>> -- 
>> 2.5.0
>> 
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@kvack.org.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me

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Thread

[PATCH V2 0/2] mm/memblock.c: fix potential bug and code refine Wei Yang <richard.weiyang@gmail.com> - 2016-12-18 15:50 +0100
  [PATCH V2 2/2] mm/memblock.c: check return value of memblock_reserve() in memblock_virt_alloc_internal() Wei Yang <richard.weiyang@gmail.com> - 2016-12-18 15:50 +0100
    Re: [PATCH V2 2/2] mm/memblock.c: check return value of  memblock_reserve() in memblock_virt_alloc_internal() Michal Hocko <mhocko@kernel.org> - 2016-12-19 16:30 +0100
      Re: [PATCH V2 2/2] mm/memblock.c: check return value of  memblock_reserve() in memblock_virt_alloc_internal() Wei Yang <richard.weiyang@gmail.com> - 2016-12-20 17:50 +0100
        Re: [PATCH V2 2/2] mm/memblock.c: check return value of  memblock_reserve() in memblock_virt_alloc_internal() Michal Hocko <mhocko@kernel.org> - 2016-12-21 09:00 +0100
          Re: [PATCH V2 2/2] mm/memblock.c: check return value of  memblock_reserve() in memblock_virt_alloc_internal() Wei Yang <richard.weiyang@gmail.com> - 2016-12-21 14:20 +0100
            Re: [PATCH V2 2/2] mm/memblock.c: check return value of  memblock_reserve() in memblock_virt_alloc_internal() Michal Hocko <mhocko@kernel.org> - 2016-12-21 14:30 +0100
              Re: [PATCH V2 2/2] mm/memblock.c: check return value of  memblock_reserve() in memblock_virt_alloc_internal() Wei Yang <richard.weiyang@gmail.com> - 2016-12-21 15:50 +0100
                Re: [PATCH V2 2/2] mm/memblock.c: check return value of  memblock_reserve() in memblock_virt_alloc_internal() Michal Hocko <mhocko@kernel.org> - 2016-12-21 16:00 +0100
  [PATCH V2 1/2] mm/memblock.c: trivial code refine in memblock_is_region_memory() Wei Yang <richard.weiyang@gmail.com> - 2016-12-18 15:50 +0100
    Re: [PATCH V2 1/2] mm/memblock.c: trivial code refine in  memblock_is_region_memory() Michal Hocko <mhocko@kernel.org> - 2016-12-19 16:20 +0100
      Re: [PATCH V2 1/2] mm/memblock.c: trivial code refine in  memblock_is_region_memory() Wei Yang <richard.weiyang@gmail.com> - 2016-12-20 17:40 +0100
        Re: [PATCH V2 1/2] mm/memblock.c: trivial code refine in  memblock_is_region_memory() Michal Hocko <mhocko@kernel.org> - 2016-12-21 08:50 +0100
          Re: [PATCH V2 1/2] mm/memblock.c: trivial code refine in  memblock_is_region_memory() Michal Hocko <mhocko@kernel.org> - 2016-12-21 13:50 +0100
            Re: [PATCH V2 1/2] mm/memblock.c: trivial code refine in  memblock_is_region_memory() Wei Yang <richard.weiyang@gmail.com> - 2016-12-21 14:20 +0100
          Re: [PATCH V2 1/2] mm/memblock.c: trivial code refine in  memblock_is_region_memory() Wei Yang <richard.weiyang@gmail.com> - 2016-12-21 13:50 +0100

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