linux-kernel - [34-longterm 038/209] mm: fix is_mem_section_removable() page_order BUG

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Message-Id: <1302803039-9400-39-git-send-email-paul.gortmaker@windriver.com>
Date:	Thu, 14 Apr 2011 13:41:08 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	stable@...nel.org, linux-kernel@...r.kernel.org
Cc:	stable-review@...nel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: [34-longterm 038/209] mm: fix is_mem_section_removable() page_order BUG_ON check

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

  =====================================================================
  | 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 <kamezawa.hiroyu@...fujitsu.com>
Acked-by: Wu Fengguang <fengguang.wu@...el.com>
Acked-by: Michal Hocko <mhocko@...e.cz>
Acked-by: Mel Gorman <mel@....ul.ie>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 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);
 }

-- 
1.7.4.4

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