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Subject[34-longterm 038/209] mm: fix is_mem_section_removable() page_order BUG_ON check
From: KAMEZAWA Hiroyuki <>

| This is a commit scheduled for the next v2.6.34 longterm release. |
| If you see a problem with using this for longterm, please comment.|

commit 572438f9b52236bd8938b1647cc15e027d27ef55 upstream.

page_order() is called by memory hotplug's user interface to check the
section is removable or not. (is_mem_section_removable())

It calls page_order() withoug holding zone->lock.
So, even if the caller does

if (PageBuddy(page))
ret = page_order(page) ...
The caller may hit BUG_ON().

For fixing this, there are 2 choices.
1. add zone->lock.
2. remove BUG_ON().

is_mem_section_removable() is used for some "advice" and doesn't need to
be 100% accurate. This is_removable() can be called via user program..
We don't want to take this important lock for long by user's request. So,
this patch removes BUG_ON().

Signed-off-by: KAMEZAWA Hiroyuki <>
Acked-by: Wu Fengguang <>
Acked-by: Michal Hocko <>
Acked-by: Mel Gorman <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Paul Gortmaker <>
mm/internal.h | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 6a697bb..dedb0af 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -62,7 +62,7 @@ extern bool is_free_buddy_page(struct page *page);
static inline unsigned long page_order(struct page *page)
- VM_BUG_ON(!PageBuddy(page));
+ /* PageBuddy() must be checked by the caller */
return page_private(page);


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