mm/page_alloc: speed up the iteration of max_order

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>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c026908b5306f132de491add958c9a7d0f1aad09..70c7cb5cc68cd8980673ed874d66325826357e52 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -996,7 +996,7 @@ static inline void __free_one_page(struct page *page,
        struct page *buddy;
        bool to_tail;
 
-       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);
@@ -1009,7 +1009,7 @@ static inline void __free_one_page(struct page *page,
        VM_BUG_ON_PAGE(bad_range(zone, page), page);
 
 continue_merging:
-       while (order < max_order - 1) {
+       while (order < max_order) {
                if (compaction_capture(capc, page, order, migratetype)) {
                        __mod_zone_freepage_state(zone, -(1 << order),
                                                                migratetype);
@@ -1035,7 +1035,7 @@ 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
@@ -1056,7 +1056,7 @@ continue_merging:
                                                is_migrate_isolate(buddy_mt)))
                                goto done_merging;
                }
-               max_order++;
+               max_order = order + 1;
                goto continue_merging;
        }