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    Subject[PATCH 4.14 024/217] mm/page_alloc: speed up the iteration of max_order
    Date
    From: Muchun Song <songmuchun@bytedance.com>

    commit 7ad69832f37e3cea8557db6df7c793905f1135e8 upstream.

    When we free a page whose order is very close to MAX_ORDER and greater
    than pageblock_order, it wastes some CPU cycles to increase max_order to
    MAX_ORDER one by one and check the pageblock migratetype of that page
    repeatedly especially when MAX_ORDER is much larger than pageblock_order.

    We also should not be checking migratetype of buddy when "order ==
    MAX_ORDER - 1" as the buddy pfn may be invalid, so adjust the condition.
    With the new check, we don't need the max_order check anymore, so we
    replace it.

    Also adjust max_order initialization so that it's lower by one than
    previously, which makes the code hopefully more clear.

    Link: https://lkml.kernel.org/r/20201204155109.55451-1-songmuchun@bytedance.com
    Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")
    Signed-off-by: Muchun Song <songmuchun@bytedance.com>
    Acked-by: Vlastimil Babka <vbabka@suse.cz>
    Reviewed-by: Oscar Salvador <osalvador@suse.de>
    Reviewed-by: David Hildenbrand <david@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    mm/page_alloc.c | 8 ++++----
    1 file changed, 4 insertions(+), 4 deletions(-)

    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -815,7 +815,7 @@ static inline void __free_one_page(struc
    struct page *buddy;
    unsigned int max_order;

    - max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
    + max_order = min_t(unsigned int, MAX_ORDER - 1, pageblock_order);

    VM_BUG_ON(!zone_is_initialized(zone));
    VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
    @@ -828,7 +828,7 @@ static inline void __free_one_page(struc
    VM_BUG_ON_PAGE(bad_range(zone, page), page);

    continue_merging:
    - while (order < max_order - 1) {
    + while (order < max_order) {
    buddy_pfn = __find_buddy_pfn(pfn, order);
    buddy = page + (buddy_pfn - pfn);

    @@ -852,7 +852,7 @@ continue_merging:
    pfn = combined_pfn;
    order++;
    }
    - if (max_order < MAX_ORDER) {
    + if (order < MAX_ORDER - 1) {
    /* If we are here, it means order is >= pageblock_order.
    * We want to prevent merge between freepages on isolate
    * pageblock and normal pageblock. Without this, pageblock
    @@ -873,7 +873,7 @@ continue_merging:
    is_migrate_isolate(buddy_mt)))
    goto done_merging;
    }
    - max_order++;
    + max_order = order + 1;
    goto continue_merging;
    }


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