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

[Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>]

On Wed, Sep 16, 2020 at 08:34:10PM +0200, 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.

I am sorry to not follow again. unset_migratetype_isolate() is used in
__offline_pages if my understanding is correct. How does it contribute on
online_pages? 

>
>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]);
>+}
>+
> 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);
> }
> 
> 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);
>-- 
>2.26.2

-- 
Wei Yang
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