From 8aea3ff411b2ce8fe7b46644298ed243a920eb24 Mon Sep 17 00:00:00 2001
From: Carlos Maiolino <cmaiolino@redhat.com>
Date: Thu, 20 Sep 2012 10:32:36 -0300
Subject: xfs: Fix m_agirotor reset during AG selection

xfs_ialloc_next_ag() currently resets m_agirotor when it is equal to
m_maxagi:

         if (++mp->m_agirotor == mp->m_maxagi)
	         mp->m_agirotor = 0;

But, if for some reason mp->m_maxagi changes to a lower value than
current m_agirotor, this condition will never be true, causing
m_agirotor to exceed the maximum allowed value (m_maxagi).

This implies mainly during lookups for xfs_perag structs in its radix
tree, since the agno value used for the lookup is based on m_agirotor.
An out-of-range m_agirotor may cause a lookup failure which in case will
return NULL.

As an example, the value of m_maxagi is decreased during
inode64->inode32 remount process, case where I've found this problem.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --git a/fs/xfs/xfs_ialloc.c b/fs/xfs/xfs_ialloc.c
index 5aceb3f..445bf1a 100644
--- a/fs/xfs/xfs_ialloc.c
+++ b/fs/xfs/xfs_ialloc.c
@@ -431,7 +431,7 @@ xfs_ialloc_next_ag(
 
 	spin_lock(&mp->m_agirotor_lock);
 	agno = mp->m_agirotor;
-	if (++mp->m_agirotor == mp->m_maxagi)
+	if (++mp->m_agirotor >= mp->m_maxagi)
 		mp->m_agirotor = 0;
 	spin_unlock(&mp->m_agirotor_lock);
 
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