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

[Vlastimil Babka <vbabka@xxxxxxx>]

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.

>  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);
>