Re: [PATCH] mm/page_alloc: reduce fallbacks to (MIGRATE_PCPTYPES - 1)

[Vlastimil Babka <vbabka@suse.cz>]

On 2/3/23 23:23, Andrew Morton wrote:
> On Fri,  3 Feb 2023 18:01:32 +0800 Yajun Deng <yajun.deng@linux.dev> wrote:
> 
>> The commit 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable
>> pageblocks with others") has removed MIGRATE_CMA and MIGRATE_ISOLATE from
>> fallbacks list. so there is no need to add an element at the end of every
>> type.
>> 
>> Reduce fallbacks to (MIGRATE_PCPTYPES - 1).


Acked-by: Vlastimil Babka <vbabka@suse.cz>

> Thanks.  `git log' suggests who should be cc'ed when fixing things...
> 
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2603,10 +2603,10 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>>   *
>>   * The other migratetypes do not have fallbacks.
>>   */
>> -static int fallbacks[MIGRATE_TYPES][3] = {
>> -	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_TYPES },
>> -	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
>> -	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_TYPES },
>> +static int fallbacks[MIGRATE_TYPES][MIGRATE_PCPTYPES - 1] = {

Why not also reduce the first index size from [MIGRATE_TYPES] to
[MIGRATE_PCPTYPES] ?

>> +	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE   },
>> +	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE },
>> +	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE   },
>>  };
>>  
>>  #ifdef CONFIG_CMA
>> @@ -2865,11 +2865,8 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
>>  		return -1;
>>  
>>  	*can_steal = false;
>> -	for (i = 0;; i++) {
>> +	for (i = 0; i < MIGRATE_PCPTYPES - 1 ; i++) {
>>  		fallback_mt = fallbacks[migratetype][i];
>> -		if (fallback_mt == MIGRATE_TYPES)
>> -			break;
>> -
>>  		if (free_area_empty(area, fallback_mt))
>>  			continue;
>>  
>> -- 
>> 2.25.1