x86/mpx: Trace the attempts to find bounds tables

There are two different events being traced here.  They are
doing similar things so share a trace "EVENT_CLASS" and are
presented together.

1. Trace when MPX is zapping pages "mpx_unmap_zap":

	When MPX can not free an entire bounds table, it will
	instead try to zap unused parts of a bounds table to free
	the backing memory.  This decreases RSS (resident set
	size) without decreasing the virtual space allocated
	for bounds tables.

2. Trace attempts to find bounds tables "mpx_unmap_search":

	This event traces any time we go looking to unmap a
	bounds table for a given virtual address range.  This is
	useful to ensure that the kernel actually "tried" to free
	a bounds table versus times it succeeded in finding one.

	It might try and fail if it realized that a table was
	shared with an adjacent VMA which is not being unmapped.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave@sr71.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150607183703.B9D2468B@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/trace/mpx.h

@@ -63,6 +63,38 @@ TRACE_EVENT(bounds_exception_mpx,
 		__entry->bndstatus)
 );
 
+DECLARE_EVENT_CLASS(mpx_range_trace,
+
+	TP_PROTO(unsigned long start,
+		 unsigned long end),
+	TP_ARGS(start, end),
+
+	TP_STRUCT__entry(
+		__field(unsigned long, start)
+		__field(unsigned long, end)
+	),
+
+	TP_fast_assign(
+		__entry->start = start;
+		__entry->end   = end;
+	),
+
+	TP_printk("[0x%p:0x%p]",
+		(void *)__entry->start,
+		(void *)__entry->end
+	)
+);
+
+DEFINE_EVENT(mpx_range_trace, mpx_unmap_zap,
+	TP_PROTO(unsigned long start, unsigned long end),
+	TP_ARGS(start, end)
+);
+
+DEFINE_EVENT(mpx_range_trace, mpx_unmap_search,
+	TP_PROTO(unsigned long start, unsigned long end),
+	TP_ARGS(start, end)
+);
+
 #else
 
 /*

arch/x86/mm/mpx.c

@@ -668,6 +668,7 @@ static int zap_bt_entries(struct mm_struct *mm,
 
 		len = min(vma->vm_end, end) - addr;
 		zap_page_range(vma, addr, len, NULL);
+		trace_mpx_unmap_zap(addr, addr+len);
 
 		vma = vma->vm_next;
 		addr = vma->vm_start;
@@ -840,6 +841,7 @@ static int mpx_unmap_tables(struct mm_struct *mm,
 	long __user *bd_entry, *bde_start, *bde_end;
 	unsigned long bt_addr;
 
+	trace_mpx_unmap_search(start, end);
 	/*
 	 * "Edge" bounds tables are those which are being used by the region
 	 * (start -> end), but that may be shared with adjacent areas.  If they