mm/damon: hide kernel pointer from tracepoint event

DAMON's virtual address spaces monitoring primitive uses 'struct pid *'
of the target process as its monitoring target id.  The kernel address
is exposed as-is to the user space via the DAMON tracepoint,
'damon_aggregated'.

Though primarily only privileged users are allowed to access that, it
would be better to avoid unnecessarily exposing kernel pointers so.
Because the trace result is only required to be able to distinguish each
target, we aren't need to use the pointer as-is.

This makes the tracepoint to use the index of the target in the
context's targets list as its id in the tracepoint, to hide the kernel
space address.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: SeongJae Park <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/trace/events/damon.h

@@ -11,10 +11,10 @@
 
 TRACE_EVENT(damon_aggregated,
 
-	TP_PROTO(struct damon_target *t, struct damon_region *r,
-		unsigned int nr_regions),
+	TP_PROTO(struct damon_target *t, unsigned int target_id,
+		struct damon_region *r, unsigned int nr_regions),
 
-	TP_ARGS(t, r, nr_regions),
+	TP_ARGS(t, target_id, r, nr_regions),
 
 	TP_STRUCT__entry(
 		__field(unsigned long, target_id)
@@ -26,7 +26,7 @@ TRACE_EVENT(damon_aggregated,
 	),
 
 	TP_fast_assign(
-		__entry->target_id = t->id;
+		__entry->target_id = target_id;
 		__entry->nr_regions = nr_regions;
 		__entry->start = r->ar.start;
 		__entry->end = r->ar.end;

mm/damon/core.c

@@ -514,15 +514,17 @@ static bool kdamond_aggregate_interval_passed(struct damon_ctx *ctx)
 static void kdamond_reset_aggregated(struct damon_ctx *c)
 {
 	struct damon_target *t;
+	unsigned int ti = 0;	/* target's index */
 
 	damon_for_each_target(t, c) {
 		struct damon_region *r;
 
 		damon_for_each_region(r, t) {
-			trace_damon_aggregated(t, r, damon_nr_regions(t));
+			trace_damon_aggregated(t, ti, r, damon_nr_regions(t));
 			r->last_nr_accesses = r->nr_accesses;
 			r->nr_accesses = 0;
 		}
+		ti++;
 	}
 }