Re: [RFC PATCH v3 2/3] mm/page_alloc: factor out fallback freepage checking

[Joonsoo Kim]

2015-02-02 21:56 GMT+09:00 Zhang Yanfei <zhangyanfei.ok@xxxxxxxxxxx>:
> Hello Joonsoo,
>
> At 2015/2/2 15:15, Joonsoo Kim wrote:
>> This is preparation step to use page allocator's anti fragmentation logic
>> in compaction. This patch just separates fallback freepage checking part
>> from fallback freepage management part. Therefore, there is no functional
>> change.
>>
>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>> ---
>>  mm/page_alloc.c | 128 +++++++++++++++++++++++++++++++++-----------------------
>>  1 file changed, 76 insertions(+), 52 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index e64b260..6cb18f8 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1142,14 +1142,26 @@ static void change_pageblock_range(struct page *pageblock_page,
>>   * as fragmentation caused by those allocations polluting movable pageblocks
>>   * is worse than movable allocations stealing from unmovable and reclaimable
>>   * pageblocks.
>> - *
>> - * If we claim more than half of the pageblock, change pageblock's migratetype
>> - * as well.
>>   */
>> -static void try_to_steal_freepages(struct zone *zone, struct page *page,
>> -                               int start_type, int fallback_type)
>> +static bool can_steal_fallback(unsigned int order, int start_mt)
>> +{
>> +     if (order >= pageblock_order)
>> +             return true;
>
> Is this test necessary? Since an order which is >= pageblock_order
> will always pass the order >= pageblock_order / 2 test below.
>

Yes, that's true. But, I'd like to remain code as is, because
condition "order >= pageblock_order / 2" is really heuristic and could
be changed someday. Instead of removing it, I will add some comment on it.

Thanks.
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