[RFC PATCH v7 05/10] trace: make the stackmap capacity settable on the kernel command line

@@ -1794,6 +1794,13 @@ Kernel parameters
 			can be changed at run time by the max_graph_depth file
 			in the tracefs tracing directory. default: 0 (no limit)
 
+	ftrace_stackmap.bits=
+			[FTRACE] Set the stackmap capacity to 2^N records.
+			Format: <int>
+			default: 14; values are clamped to the range 10-18.
+			The upper bound reserves roughly 130 MB for the element
+			pool. See Documentation/trace/ftrace-stackmap.rst.
+
 	fw_devlink=	[KNL,EARLY] Create device links between consumer and supplier
 			devices by scanning the firmware to infer the
 			consumer/supplier relationships. This feature is

@@ -435,6 +435,10 @@ config FTRACE_STACKMAP
 	  deduplicated when stackmap is enabled for their trace array. Stackmap
 	  is currently available only for the global trace array.
 
+	  Map capacity defaults to 2^14 records and can be changed with the
+	  ftrace_stackmap.bits= boot parameter. Values are clamped to 10-18;
+	  the upper bound reserves roughly 130 MB for the element pool.
+
 	  The deduplicated stacks are exported via:
 	    /sys/kernel/debug/tracing/stack_map
 

@@ -150,12 +150,31 @@ struct ftrace_stackmap {
 };
 
 /*
- * Map capacity: 2^FTRACE_STACKMAP_BITS stack records, with the hash
- * table over-provisioned 2x on top of that. This gives 16K elements
- * and a ~8 MB element pool, sized for the repetitive-stack workloads
- * the map targets.
+ * Map capacity: 2^bits stack records, with the hash table
+ * over-provisioned 2x on top of that. The range is capped to keep
+ * worst-case allocations bounded:
+ *   bits=18 -> 256K elts, 512K slots, ~130 MB elt pool, and a
+ *              stack_map_bin reader walking that many entries.
+ * The default of 14 gives 16K elts and a ~8 MB pool, which suits the
+ * repetitive-stack workloads the map targets; raise it through
+ * ftrace_stackmap.bits= for a higher stack-record capacity.
  */
-#define FTRACE_STACKMAP_BITS		14
+#define FTRACE_STACKMAP_BITS_MIN	10
+#define FTRACE_STACKMAP_BITS_MAX	18
+#define FTRACE_STACKMAP_BITS_DEFAULT	14
+
+static unsigned int stackmap_map_bits = FTRACE_STACKMAP_BITS_DEFAULT;
+static int __init stackmap_bits_setup(char *str)
+{
+	unsigned long val;
+
+	if (kstrtoul(str, 0, &val))
+		return -EINVAL;
+	val = clamp_val(val, FTRACE_STACKMAP_BITS_MIN, FTRACE_STACKMAP_BITS_MAX);
+	stackmap_map_bits = val;
+	return 0;
+}
+early_param("ftrace_stackmap.bits", stackmap_bits_setup);
 
 /* --- Element pool --- */
 

@@ -182,12 +201,17 @@ static struct stackmap_elt *stackmap_get_elt(struct ftrace_stackmap *smap)
 struct ftrace_stackmap *ftrace_stackmap_create(struct trace_array *tr)
 {
 	struct ftrace_stackmap *smap;
-	unsigned int bits = FTRACE_STACKMAP_BITS;
+	unsigned int bits;
 
 	smap = kzalloc_obj(*smap, GFP_KERNEL);
 	if (!smap)
 		return ERR_PTR(-ENOMEM);
 
+	/* Defensive clamp even if the early parameter path is bypassed. */
+	bits = clamp_val(stackmap_map_bits,
+			 FTRACE_STACKMAP_BITS_MIN,
+			 FTRACE_STACKMAP_BITS_MAX);
+
 	smap->tr = tr;
 	smap->map_bits = bits;
 	smap->max_elts = 1U << bits;