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Subject[PATCH 3.17 07/47] slab: fix nodeid bounds check for non-contiguous node IDs
3.17-stable review patch.  If anyone has any objections, please let me know.


From: Paul Mackerras <>

commit 7c3fbbdd04a681a1992ad6a3d7a36a63ff668753 upstream.

The bounds check for nodeid in ____cache_alloc_node gives false
positives on machines where the node IDs are not contiguous, leading to
a panic at boot time. For example, on a POWER8 machine the node IDs are
typically 0, 1, 16 and 17. This means that num_online_nodes() returns
4, so when ____cache_alloc_node is called with nodeid = 16 the VM_BUG_ON
triggers, like this:

kernel BUG at /home/paulus/kernel/kvm/mm/slab.c:3079!
Call Trace:
.____cache_alloc_node+0x5c/0x270 (unreliable)

To fix this, we instead compare the nodeid with MAX_NUMNODES, and
additionally make sure it isn't negative (since nodeid is an int). The
check is there mainly to protect the array dereference in the get_node()
call in the next line, and the array being dereferenced is of size
MAX_NUMNODES. If the nodeid is in range but invalid (for example if the
node is off-line), the BUG_ON in the next line will catch that.

Fixes: 14e50c6a9bc2 ("mm: slab: Verify the nodeid passed to ____cache_alloc_node")
Signed-off-by: Paul Mackerras <>
Reviewed-by: Yasuaki Ishimatsu <>
Reviewed-by: Pekka Enberg <>
Acked-by: David Rientjes <>
Cc: Christoph Lameter <>
Cc: Joonsoo Kim <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

mm/slab.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3108,7 +3108,7 @@ static void *____cache_alloc_node(struct
void *obj;
int x;

- VM_BUG_ON(nodeid > num_online_nodes());
+ VM_BUG_ON(nodeid < 0 || nodeid >= MAX_NUMNODES);
n = get_node(cachep, nodeid);

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