linux-kernel - Re: [PATCH] mm: remove a redundant condition in the for loop

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Date:   Mon, 19 Jun 2017 21:05:29 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Hao Lee <haolee.swjtu@...il.com>, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, mhocko@...e.com, hannes@...xchg.org,
        iamjoonsoo.kim@....com, minchan@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: remove a redundant condition in the for loop

On Mon, Jun 19 2017, Vlastimil Babka <vbabka@...e.cz> wrote:

> On 06/19/2017 03:54 PM, Hao Lee wrote:
>> The variable current_order decreases from MAX_ORDER-1 to order, so the
>> condition current_order <= MAX_ORDER-1 is always true.
>> 
>> Signed-off-by: Hao Lee <haolee.swjtu@...il.com>
>
> Sounds right.
>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

current_order and order are both unsigned, and if order==0,
current_order >= order is always true, and we may decrement
current_order past 0 making it UINT_MAX... A comment would be in order,
though.

>> ---
>>  mm/page_alloc.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2302f25..9120c2b 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
>>  	bool can_steal;
>>  
>>  	/* Find the largest possible block of pages in the other list */
>> -	for (current_order = MAX_ORDER-1;
>> -				current_order >= order && current_order <= MAX_ORDER-1;
>> -				--current_order) {
>> +	for (current_order = MAX_ORDER-1; current_order >= order;
>> +							--current_order) {
>>  		area = &(zone->free_area[current_order]);
>>  		fallback_mt = find_suitable_fallback(area, current_order,
>>  				start_migratetype, false, &can_steal);
>>