Re: [RFC PATCH] mm/zsmalloc: remove unnecessary check

[Ganesh Mahendran]

Hello

2014-11-21 11:54 GMT+08:00 Minchan Kim <minchan@xxxxxxxxxx>:
> On Thu, Nov 20, 2014 at 09:21:56PM +0800, Mahendran Ganesh wrote:
>> ZS_SIZE_CLASSES is calc by:
>>   ((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / ZS_SIZE_CLASS_DELTA + 1)
>>
>> So when i is in [0, ZS_SIZE_CLASSES - 1), the size:
>>   size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA
>> will not be greater than ZS_MAX_ALLOC_SIZE
>>
>> This patch removes the unnecessary check.
>
> It depends on ZS_MIN_ALLOC_SIZE.
> For example, we would change min to 8 but MAX is still 4096.
> ZS_SIZE_CLASSES is (4096 - 8) / 16 + 1 = 256 so 8 + 255 * 16 = 4088,
> which exceeds the max.
Here, 4088 is less than MAX(4096).

ZS_SIZE_CLASSES = (MAX - MIN) / Delta + 1
So, I think the value of
    MIN + (ZS_SIZE_CLASSES - 1) * Delta =
    MIN + ((MAX - MIN) / Delta) * Delta =
    MAX
will not exceed the MAX

Thanks.

>
>>
>> Signed-off-by: Mahendran Ganesh <opensource.ganesh@xxxxxxxxx>
>> ---
>>  mm/zsmalloc.c |    2 --
>>  1 file changed, 2 deletions(-)
>>
>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>> index b3b57ef..f2279e2 100644
>> --- a/mm/zsmalloc.c
>> +++ b/mm/zsmalloc.c
>> @@ -973,8 +973,6 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>>               struct size_class *prev_class;
>>
>>               size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
>> -             if (size > ZS_MAX_ALLOC_SIZE)
>> -                     size = ZS_MAX_ALLOC_SIZE;
>>               pages_per_zspage = get_pages_per_zspage(size);
>>
>>               /*
>> --
>> 1.7.9.5
>>
>> --
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>
> --
> Kind regards,
> Minchan Kim
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