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[PATCH 1/6] xfs: Fix m_agirotor reset during AG selection

To: xfs@xxxxxxxxxxx
Subject: [PATCH 1/6] xfs: Fix m_agirotor reset during AG selection
From: Carlos Maiolino <cmaiolino@xxxxxxxxxx>
Date: Thu, 20 Sep 2012 10:32:36 -0300
In-reply-to: <1348147961-7520-1-git-send-email-cmaiolino@xxxxxxxxxx>
References: <1348147961-7520-1-git-send-email-cmaiolino@xxxxxxxxxx>
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@xxxxxxxxxx>
---
 fs/xfs/xfs_ialloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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);
 
-- 
1.7.11.4

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