Re: [PATCH v1 3/5] mm/page_alloc: always move pages to the tail of the freelist in unset_migratetype_isolate()

[Michal Hocko <mhocko@suse.com>]

On Mon 28-09-20 20:21:08, 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.
> 
> Add a "to_tail" parameter to move_freepages_block() but introduce a
> a new move_to_free_list_tail() - similar to add_to_free_list_tail().
> 
> This change results in all pages getting onlined via online_pages() to
> be placed to the tail of the freelist.

Is there anything preventing to do this unconditionally? Or in other
words is any of the existing callers of move_freepages_block benefiting
from adding to the head?

> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Scott Cheloha <cheloha@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/linux/page-isolation.h |  4 ++--
>  mm/page_alloc.c                | 35 +++++++++++++++++++++++-----------
>  mm/page_isolation.c            | 12 +++++++++---
>  3 files changed, 35 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
> index 572458016331..3eca9b3c5305 100644
> --- a/include/linux/page-isolation.h
> +++ b/include/linux/page-isolation.h
> @@ -36,8 +36,8 @@ static inline bool is_migrate_isolate(int migratetype)
>  struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  				 int migratetype, int flags);
>  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(struct zone *zone, struct page *page, int migratetype,
> +			 bool to_tail, int *num_movable);
>  
>  /*
>   * 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 9e3ed4a6f69a..d5a5f528b8ca 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -905,6 +905,15 @@ 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]);
> +}
> +
>  static inline void del_page_from_free_list(struct page *page, struct zone *zone,
>  					   unsigned int order)
>  {
> @@ -2338,9 +2347,9 @@ static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
>   * Note that start_page and end_pages are not aligned on a pageblock
>   * boundary. If alignment is required, use move_freepages_block()
>   */
> -static int move_freepages(struct zone *zone,
> -			  struct page *start_page, struct page *end_page,
> -			  int migratetype, int *num_movable)
> +static int move_freepages(struct zone *zone, struct page *start_page,
> +			  struct page *end_page, int migratetype,
> +			  bool to_tail, int *num_movable)
>  {
>  	struct page *page;
>  	unsigned int order;
> @@ -2371,7 +2380,10 @@ 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;
>  	}
> @@ -2379,8 +2391,8 @@ 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)
> +int move_freepages_block(struct zone *zone, struct page *page, int migratetype,
> +			 bool to_tail, int *num_movable)
>  {
>  	unsigned long start_pfn, end_pfn;
>  	struct page *start_page, *end_page;
> @@ -2401,7 +2413,7 @@ int move_freepages_block(struct zone *zone, struct page *page,
>  		return 0;
>  
>  	return move_freepages(zone, start_page, end_page, migratetype,
> -								num_movable);
> +			      to_tail, num_movable);
>  }
>  
>  static void change_pageblock_range(struct page *pageblock_page,
> @@ -2526,8 +2538,8 @@ static void steal_suitable_fallback(struct zone *zone, struct page *page,
>  	if (!whole_block)
>  		goto single_page;
>  
> -	free_pages = move_freepages_block(zone, page, start_type,
> -						&movable_pages);
> +	free_pages = move_freepages_block(zone, page, start_type, false,
> +					  &movable_pages);
>  	/*
>  	 * Determine how many pages are compatible with our allocation.
>  	 * For movable allocation, it's the number of movable pages which
> @@ -2635,7 +2647,8 @@ static void reserve_highatomic_pageblock(struct page *page, struct zone *zone,
>  	    && !is_migrate_cma(mt)) {
>  		zone->nr_reserved_highatomic += pageblock_nr_pages;
>  		set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
> -		move_freepages_block(zone, page, MIGRATE_HIGHATOMIC, NULL);
> +		move_freepages_block(zone, page, MIGRATE_HIGHATOMIC, false,
> +				     NULL);
>  	}
>  
>  out_unlock:
> @@ -2711,7 +2724,7 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
>  			 */
>  			set_pageblock_migratetype(page, ac->migratetype);
>  			ret = move_freepages_block(zone, page, ac->migratetype,
> -									NULL);
> +						   false, NULL);
>  			if (ret) {
>  				spin_unlock_irqrestore(&zone->lock, flags);
>  				return ret;
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index abfe26ad59fd..de44e1329706 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -45,7 +45,7 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>  		set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>  		zone->nr_isolate_pageblock++;
>  		nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE,
> -									NULL);
> +						false, NULL);
>  
>  		__mod_zone_freepage_state(zone, -nr_pages, mt);
>  		spin_unlock_irqrestore(&zone->lock, flags);
> @@ -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 +106,15 @@ 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 freelist. This is an optimization for memory
> +	 * onlining - just onlined memory won't immediately be considered for
> +	 * allocation.
>  	 */
>  	if (!isolated_page) {
> -		nr_pages = move_freepages_block(zone, page, migratetype, NULL);
> +		nr_pages = move_freepages_block(zone, page, migratetype, true,
> +						NULL);
>  		__mod_zone_freepage_state(zone, nr_pages, migratetype);
>  	}
>  	set_pageblock_migratetype(page, migratetype);
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs