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

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