Re: [PATCH RFC 3/4] mm/page_alloc: always move pages to the tail of the freelist in unset_migratetype_isolate()

[Wei Yang]

On Thu, Sep 24, 2020 at 01:13:29PM +0200, Vlastimil Babka wrote:
>On 9/16/20 8:34 PM, David Hildenbrand wrote:
>> Page isolation doesn't actually touch the pages, it simply isolates
>> pageblocks and moves all free pages to the MIGRATE_ISOLATE freelist.
>> 
>> We already place pages to the tail of the freelists when undoing
>> isolation via __putback_isolated_page(), let's do it in any case
>> (e.g., if order == pageblock_order) and document the behavior.
>> 
>> This change results in all pages getting onlined via online_pages() to
>> be placed to the tail of the freelist.
>> 
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
>> Cc: Vlastimil Babka <vbabka@xxxxxxx>
>> Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
>> Cc: Oscar Salvador <osalvador@xxxxxxx>
>> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
>> Cc: Scott Cheloha <cheloha@xxxxxxxxxxxxx>
>> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>> ---
>>  include/linux/page-isolation.h |  2 ++
>>  mm/page_alloc.c                | 36 +++++++++++++++++++++++++++++-----
>>  mm/page_isolation.c            |  8 ++++++--
>>  3 files changed, 39 insertions(+), 7 deletions(-)
>> 
>> diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
>> index 572458016331..a36be2cf4dbb 100644
>> --- a/include/linux/page-isolation.h
>> +++ b/include/linux/page-isolation.h
>> @@ -38,6 +38,8 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>  void set_pageblock_migratetype(struct page *page, int migratetype);
>>  int move_freepages_block(struct zone *zone, struct page *page,
>>  				int migratetype, int *num_movable);
>> +int move_freepages_block_tail(struct zone *zone, struct page *page,
>> +			      int migratetype);
>>  
>>  /*
>>   * Changes migrate type in [start_pfn, end_pfn) to be MIGRATE_ISOLATE.
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index bba9a0f60c70..75b0f49b4022 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -899,6 +899,15 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
>>  	list_move(&page->lru, &area->free_list[migratetype]);
>>  }
>>  
>> +/* Used for pages which are on another list */
>> +static inline void move_to_free_list_tail(struct page *page, struct zone *zone,
>> +					  unsigned int order, int migratetype)
>> +{
>> +	struct free_area *area = &zone->free_area[order];
>> +
>> +	list_move_tail(&page->lru, &area->free_list[migratetype]);
>> +}
>
>There are just 3 callers of move_to_free_list() before this patch, I would just
>add the to_tail parameter there instead of new wrapper. For callers with
>constant parameter, the inline will eliminate it anyway.

Got the same feeling :-)

>
>>  static inline void del_page_from_free_list(struct page *page, struct zone *zone,
>>  					   unsigned int order)
>>  {
>> @@ -2323,7 +2332,7 @@ static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
>>   */
>>  static int move_freepages(struct zone *zone,
>>  			  struct page *start_page, struct page *end_page,
>> -			  int migratetype, int *num_movable)
>> +			  int migratetype, int *num_movable, bool to_tail)
>>  {
>>  	struct page *page;
>>  	unsigned int order;
>> @@ -2354,7 +2363,10 @@ static int move_freepages(struct zone *zone,
>>  		VM_BUG_ON_PAGE(page_zone(page) != zone, page);
>>  
>>  		order = page_order(page);
>> -		move_to_free_list(page, zone, order, migratetype);
>> +		if (to_tail)
>> +			move_to_free_list_tail(page, zone, order, migratetype);
>> +		else
>> +			move_to_free_list(page, zone, order, migratetype);
>>  		page += 1 << order;
>>  		pages_moved += 1 << order;
>>  	}
>> @@ -2362,8 +2374,9 @@ static int move_freepages(struct zone *zone,
>>  	return pages_moved;
>>  }
>>  
>> -int move_freepages_block(struct zone *zone, struct page *page,
>> -				int migratetype, int *num_movable)
>> +static int __move_freepages_block(struct zone *zone, struct page *page,
>> +				  int migratetype, int *num_movable,
>> +				  bool to_tail)
>>  {
>>  	unsigned long start_pfn, end_pfn;
>>  	struct page *start_page, *end_page;
>> @@ -2384,7 +2397,20 @@ int move_freepages_block(struct zone *zone, struct page *page,
>>  		return 0;
>>  
>>  	return move_freepages(zone, start_page, end_page, migratetype,
>> -								num_movable);
>> +			      num_movable, to_tail);
>> +}
>> +
>> +int move_freepages_block(struct zone *zone, struct page *page,
>> +			 int migratetype, int *num_movable)
>> +{
>> +	return __move_freepages_block(zone, page, migratetype, num_movable,
>> +				      false);
>> +}
>> +
>> +int move_freepages_block_tail(struct zone *zone, struct page *page,
>> +			      int migratetype)
>> +{
>> +	return __move_freepages_block(zone, page, migratetype, NULL, true);
>>  }
>
>Likewise, just 5 callers of move_freepages_block(), all in the files you're
>already changing, so no need for this wrappers IMHO.
>
>Thanks,
>Vlastimil
>
>>  static void change_pageblock_range(struct page *pageblock_page,
>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> index abfe26ad59fd..84aa1d14751d 100644
>> --- a/mm/page_isolation.c
>> +++ b/mm/page_isolation.c
>> @@ -83,7 +83,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>  	 * Because freepage with more than pageblock_order on isolated
>>  	 * pageblock is restricted to merge due to freepage counting problem,
>>  	 * it is possible that there is free buddy page.
>> -	 * move_freepages_block() doesn't care of merge so we need other
>> +	 * move_freepages_block*() don't care about merging, so we need another
>>  	 * approach in order to merge them. Isolation and free will make
>>  	 * these pages to be merged.
>>  	 */
>> @@ -106,9 +106,13 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>  	 * If we isolate freepage with more than pageblock_order, there
>>  	 * should be no freepage in the range, so we could avoid costly
>>  	 * pageblock scanning for freepage moving.
>> +	 *
>> +	 * We didn't actually touch any of the isolated pages, so place them
>> +	 * to the tail of the freelists. This is especially relevant during
>> +	 * memory onlining.
>>  	 */
>>  	if (!isolated_page) {
>> -		nr_pages = move_freepages_block(zone, page, migratetype, NULL);
>> +		nr_pages = move_freepages_block_tail(zone, page, migratetype);
>>  		__mod_zone_freepage_state(zone, nr_pages, migratetype);
>>  	}
>>  	set_pageblock_migratetype(page, migratetype);
>> 

-- 
Wei Yang
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