Re: [RESEND][PATCH] __isolate_lru_page:skip unneeded "not"

[Bob Liu <lliubbo@gmail.com>]

On 4/3/10, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Thu,  1 Apr 2010 21:37:35 +0800
> Bob Liu <lliubbo@gmail.com> wrote:
>
>> PageActive(page) will return int 0 or 1, mode is also int 0 or 1,
>> they are comparible so "not" is unneeded to be sure to boolean
>> values.
>> I also collected the ISOLATE_BOTH check together.
>>
>> Signed-off-by: Bob Liu <lliubbo@gmail.com>
>> ---
>>  mm/vmscan.c |   15 +++++----------
>>  1 files changed, 5 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index e0e5f15..ce9ee85 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -862,16 +862,11 @@ int __isolate_lru_page(struct page *page, int mode,
>> int file)
>>  	if (!PageLRU(page))
>>  		return ret;
>>
>> -	/*
>> -	 * When checking the active state, we need to be sure we are
>> -	 * dealing with comparible boolean values.  Take the logical not
>> -	 * of each.
>> -	 */
>
> You deleted a spelling mistake too!
>
>> -	if (mode != ISOLATE_BOTH && (!PageActive(page) != !mode))
>> -		return ret;
>> -
>> -	if (mode != ISOLATE_BOTH && page_is_file_cache(page) != file)
>> -		return ret;
>> +	if (mode != ISOLATE_BOTH) {
>> +		if ((PageActive(page) != mode) ||
>> +			(page_is_file_cache(page) != file))
>> +				return ret;
>> +	}
>
> The compiler should be able to avoid testing for ISOLATE_BOTH twice,

Thanks for your kindly reply.
then is the two "not" able to avoid by the compiler ?
if yes, this patch is meanless and should be ignore.

> and I think the previous code layout was superior:
>
> 	if (mode != ISOLATE_BOTH && (!PageActive(page) != !mode))
> 		return ret;
>
> 	if (mode != ISOLATE_BOTH && page_is_file_cache(page) != file)
> 		return ret;
>
> Because it gives us nice places to put a comment explaining what the
> code is doing, whereas making it a more complex single expression:
>
> 	if (mode != ISOLATE_BOTH) {
> 		if ((PageActive(page) != mode) ||
> 			(page_is_file_cache(page) != file))
> 				return ret;
> 	}
>
> makes clearly commenting each test more difficult.
>
> Yeah, there's no comment there at present.  But that's because we suck
> - I'm sure someone is working on it ;)
>
>
-- 
Regards,
--Bob

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