Like below, memory-hotplug makes race between page-isolation
and page-allocation so it can hit BUG_ON in __offline_isolated_pages.

        CPU A                                   CPU B

start_isolate_page_range
set_migratetype_isolate
spin_lock_irqsave(zone->lock)

                                free_hot_cold_page(Page A)
                                /* without zone->lock */
                                migratetype = get_pageblock_migratetype(Page A);
                                /*
                                 * Page could be moved into MIGRATE_MOVABLE
                                 * of per_cpu_pages
                                 */
                                list_add_tail(&page->lru, 
&pcp->lists[migratetype]);

set_pageblock_isolate
move_freepages_block
drain_all_pages

                                /* Page A could be in MIGRATE_MOVABLE of 
free_list. */

check_pages_isolated
__test_page_isolated_in_pageblock
/*
 * We can't catch freed page which
 * is free_list[MIGRATE_MOVABLE]
 */
if (PageBuddy(page A))
        pfn += 1 << page_order(page A);

                                /* So, Page A could be allocated */

__offline_isolated_pages
/*
 * BUG_ON hit or offline page
 * which is used by someone
 */
BUG_ON(!PageBuddy(page A));

Signed-off-by: Minchan Kim <minc...@kernel.org>
---
I found this problem during code review so please confirm it.
Kame?

 mm/page_isolation.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index acf65a7..4699d1f 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -196,8 +196,11 @@ __test_page_isolated_in_pageblock(unsigned long pfn, 
unsigned long end_pfn)
                        continue;
                }
                page = pfn_to_page(pfn);
-               if (PageBuddy(page))
+               if (PageBuddy(page)) {
+                       if (get_page_migratetype(page) != MIGRATE_ISOLATE)
+                               break;
                        pfn += 1 << page_order(page);
+               }
                else if (page_count(page) == 0 &&
                                get_page_migratetype(page) == MIGRATE_ISOLATE)
                        pfn += 1;
-- 
1.7.9.5

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