[PATCH] tracing: Call vmalloc_sync_mappings() after alloc_percpu()

[Joerg Roedel]

Tracing code allocates percpu memory which is touched when trace events
happen. When the events happen in the page-fault handler and per-cpu
memory needs to be faulted in, the page-fault on per-cpu memory might
never complete.

Call vmalloc_sync_mappings() after allocating the runtime per-cpu
buffers to make sure they are mapped when the page-fault handler tries
to access them.

This is not a nice solution, but the best fix until the
vmalloc_sync_mappings/unmappings() interface can be ripped out of the
kernel.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 kernel/trace/trace.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8d2b98812625..0fc56063fcca 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8525,6 +8525,19 @@ static int allocate_trace_buffers(struct trace_array *tr, int size)
 	 */
 	allocate_snapshot = false;
 #endif
+
+	/*
+	 * Because of some magic with the way alloc_percpu() works on
+	 * x86_64, we need to synchronize the pgd of all the tables,
+	 * otherwise the trace events that happen in x86_64 page fault
+	 * handlers can't cope with accessing the chance that a
+	 * alloc_percpu()'d memory might be touched in the page fault trace
+	 * event. Oh, and we need to audit all alloc_percpu() and vmalloc()
+	 * calls in tracing, because something might get triggered within a
+	 * page fault trace event!
+	 */
+	vmalloc_sync_mappings();
+
 	return 0;
 }
 
-- 
2.12.3