Re: [RFC PATCH 1/3 V1] mm, page_alloc: use __rmqueue_smallest when borrow memory from MIGRATE_CMA

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 07/04/2012 06:17 PM, Mel Gorman wrote:
> On Wed, Jul 04, 2012 at 03:26:16PM +0800, Lai Jiangshan wrote:
>> The pages of MIGRATE_CMA can't not be changed to the other type,
>> nor be moved to the other free list. 
>>
>> ==>
>> So when we use __rmqueue_fallback() to borrow memory from MIGRATE_CMA,
>> one of the highest order page is borrowed and it is split.
>> But the free pages resulted by splitting can NOT
>> be moved to MIGRATE_MOVABLE.
>>
>> ==>
>> So in the next time of allocation, we NEED to borrow again,
>> another one of the highest order page is borrowed from CMA and it is split.
>> and results some other new split free pages.
>>
> 
> Then special case __rmqueue_fallback() to move pages stolen from
> MIGRATE_CMA to the MIGRATE_MOVABLE lists but do not change the pageblock
> type.

Because unmovable-page-requirement can allocate page from
MIGRATE_MOVABLE free list. So We can not move MIGRATE_CMA pages
to the MIGRATE_MOVABLE free list.

See here:

MOVABLE list is empty
UNMOVABLE list is empty
movable-page-requirement
	borrow from CMA list
	split it, others are put into UNMOVABLE list
unmovable-page-requiremnt
	borrow from UNMOVABLE list
	NOW, it is BUG, we use CMA pages for unmovable usage.



> 
>> ==>
>> So when __rmqueue_fallback() borrows (highest order)memory from MIGRATE_CMA,
>> it introduces fragments at the same time and may waste tlb(only one page is used in
>> a pageblock).
>>
>> Conclusion:
>> We should borrows the smallest order memory from MIGRATE_CMA in such case
>>
> 
> That's excessive and unnecessary special casing. Just move the
> MIGRATE_CMA pages to the MIGRATE_MOVABLE lists.

...

> 
>> Result(good):
>> 1) use __rmqueue_smallest when borrow memory from MIGRATE_CMA
>> 2) __rmqueue_fallback() don't handle CMA, it becomes much simpler
>> Result(bad):
>> __rmqueue_smallest() can't not be inlined to avoid function call overhead.
>>
>> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
>> ---
>>  include/linux/mmzone.h |    1 +
>>  mm/page_alloc.c        |   63 ++++++++++++++++--------------------------------
>>  2 files changed, 22 insertions(+), 42 deletions(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index bf3404e..979c333 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -40,6 +40,7 @@ enum {
>>  	MIGRATE_RECLAIMABLE,
>>  	MIGRATE_MOVABLE,
>>  	MIGRATE_PCPTYPES,	/* the number of types on the pcp lists */
>> +	MIGRATE_PRIME_TYPES = MIGRATE_PCPTYPES,
> 
> No explanation why this new name is necessary.

These three types are the basic types.
reusing the name MIGRATE_PCPTYPES in fallback array makes confusing.


> 
>>  	MIGRATE_RESERVE = MIGRATE_PCPTYPES,
>>  #ifdef CONFIG_CMA
>>  	/*
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 476ae3e..efc327f 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -893,17 +893,10 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>>   * This array describes the order lists are fallen back to when
>>   * the free lists for the desirable migrate type are depleted
>>   */
>> -static int fallbacks[MIGRATE_TYPES][4] = {
>> -	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,     MIGRATE_RESERVE },
>> -	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,     MIGRATE_RESERVE },
>> -#ifdef CONFIG_CMA
>> -	[MIGRATE_MOVABLE]     = { MIGRATE_CMA,         MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
>> -	[MIGRATE_CMA]         = { MIGRATE_RESERVE }, /* Never used */
>> -#else
>> -	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE,   MIGRATE_RESERVE },
>> -#endif
>> -	[MIGRATE_RESERVE]     = { MIGRATE_RESERVE }, /* Never used */
>> -	[MIGRATE_ISOLATE]     = { MIGRATE_RESERVE }, /* Never used */
>> +static int fallbacks[MIGRATE_PRIME_TYPES][2] = {
>> +	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE   },
>> +	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE   },
>> +	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE },
>>  };
>>  
> 
> And it's completely unclear why it was necessary to rip out the existing
> fallback lists. It reworks how fallback lists work for no clear benefit.
> 
>>  /*
>> @@ -995,16 +988,15 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>>  	struct page *page;
>>  	int migratetype, i;
>>  
>> +	if (WARN_ON_ONCE(start_migratetype >= MIGRATE_PRIME_TYPES))
>> +		start_migratetype = MIGRATE_UNMOVABLE;
>> +
> 
> This should be completely unnecessary. If this warning is hit, the
> callers are severely broken.

will be removed.

> 
>>  	/* Find the largest possible block of pages in the other list */
>>  	for (current_order = MAX_ORDER-1; current_order >= order;
>>  						--current_order) {
>> -		for (i = 0;; i++) {
>> +		for (i = 0; i < ARRAY_SIZE(fallbacks[0]); i++) {
>>  			migratetype = fallbacks[start_migratetype][i];
>>  
>> -			/* MIGRATE_RESERVE handled later if necessary */
>> -			if (migratetype == MIGRATE_RESERVE)
>> -				break;
>> -
>>  			area = &(zone->free_area[current_order]);
>>  			if (list_empty(&area->free_list[migratetype]))
>>  				continue;
>> @@ -1018,17 +1010,10 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>>  			 * pages to the preferred allocation list. If falling
>>  			 * back for a reclaimable kernel allocation, be more
>>  			 * aggressive about taking ownership of free pages
>> -			 *
>> -			 * On the other hand, never change migration
>> -			 * type of MIGRATE_CMA pageblocks nor move CMA
>> -			 * pages on different free lists. We don't
>> -			 * want unmovable pages to be allocated from
>> -			 * MIGRATE_CMA areas.
>>  			 */
>> -			if (!is_migrate_cma(migratetype) &&
>> -			    (unlikely(current_order >= pageblock_order / 2) ||
>> -			     start_migratetype == MIGRATE_RECLAIMABLE ||
>> -			     page_group_by_mobility_disabled)) {
>> +			if (unlikely(current_order >= pageblock_order / 2) ||
>> +			    start_migratetype == MIGRATE_RECLAIMABLE ||
>> +			    page_group_by_mobility_disabled) {
>>  				int pages;
>>  				pages = move_freepages_block(zone, page,
>>  								start_migratetype);
>> @@ -1047,14 +1032,12 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>>  			rmv_page_order(page);
>>  
>>  			/* Take ownership for orders >= pageblock_order */
>> -			if (current_order >= pageblock_order &&
>> -			    !is_migrate_cma(migratetype))
>> +			if (current_order >= pageblock_order)
>>  				change_pageblock_range(page, current_order,
>>  							start_migratetype);
>>  
>>  			expand(zone, page, order, current_order, area,
>> -			       is_migrate_cma(migratetype)
>> -			     ? migratetype : start_migratetype);
>> +			       start_migratetype);
>>  
>>  			trace_mm_page_alloc_extfrag(page, order, current_order,
>>  				start_migratetype, migratetype);
>> @@ -1075,22 +1058,18 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
>>  {
>>  	struct page *page;
>>  
>> -retry_reserve:
>>  	page = __rmqueue_smallest(zone, order, migratetype);
>>  
>> -	if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
>> +#ifdef CONFIG_CMA
>> +	if (unlikely(!page) && migratetype == MIGRATE_MOVABLE)
>> +		page = __rmqueue_smallest(zone, order, MIGRATE_CMA);
>> +#endif
>> +
>> +	if (unlikely(!page))
>>  		page = __rmqueue_fallback(zone, order, migratetype);
>>  
>> -		/*
>> -		 * Use MIGRATE_RESERVE rather than fail an allocation. goto
>> -		 * is used because __rmqueue_smallest is an inline function
>> -		 * and we want just one call site
>> -		 */
>> -		if (!page) {
>> -			migratetype = MIGRATE_RESERVE;
>> -			goto retry_reserve;
>> -		}
>> -	}
>> +	if (unlikely(!page))
>> +		page = __rmqueue_smallest(zone, order, MIGRATE_RESERVE);
>>  
>>  	trace_mm_page_alloc_zone_locked(page, order, migratetype);
>>  	return page;
> 
> I didn't read this closely. It seems way more complex than necessary to
> solve the problem described in the changelog. All you should need is to
> special case that a __GFP_MOVABLE allocation using MIGRATE_CMA should
> place the unused buddies on the MIGRATE_MOVABLE lists.
>