linux-kernel - [ 07/53] hugetlb: prevent BUG_ON in hugetlb_fault() -> hugetlb

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Message-Id: <20120518023255.315413519@decadent.org.uk>
Date:	Fri, 18 May 2012 03:33:01 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Chris Metcalf <cmetcalf@...era.com>,
	Mel Gorman <mel@....ul.ie>, Hillf Danton <dhillf@...il.com>,
	Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: [ 07/53] hugetlb: prevent BUG_ON in hugetlb_fault() -> hugetlb_cow()

3.2.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Chris Metcalf <cmetcalf@...era.com>

commit 4998a6c0edce7fae9c0a5463f6ec3fa585258ee7 upstream.

Commit 66aebce747eaf ("hugetlb: fix race condition in hugetlb_fault()")
added code to avoid a race condition by elevating the page refcount in
hugetlb_fault() while calling hugetlb_cow().

However, one code path in hugetlb_cow() includes an assertion that the
page count is 1, whereas it may now also have the value 2 in this path.

The consensus is that this BUG_ON has served its purpose, so rather than
extending it to cover both cases, we just remove it.

Signed-off-by: Chris Metcalf <cmetcalf@...era.com>
Acked-by: Mel Gorman <mel@....ul.ie>
Acked-by: Hillf Danton <dhillf@...il.com>
Acked-by: Hugh Dickins <hughd@...gle.com>
Cc: Michal Hocko <mhocko@...e.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/hugetlb.c |    1 -
 1 file changed, 1 deletion(-)

--- linux.orig/mm/hugetlb.c
+++ linux/mm/hugetlb.c
@@ -2405,7 +2405,6 @@
 		if (outside_reserve) {
 			BUG_ON(huge_pte_none(pte));
 			if (unmap_ref_private(mm, vma, old_page, address)) {
-				BUG_ON(page_count(old_page) != 1);
 				BUG_ON(huge_pte_none(pte));
 				spin_lock(&mm->page_table_lock);
 				goto retry_avoidcopy;



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