fgraph: Give ret_stack its own kmem cache

The ret_stack (shadow stack used by function graph infrastructure) is
created for every task on the system when function graph is enabled. Give
it its own kmem_cache. This will make it easier to see how much memory is
being used specifically for function graph shadow stacks.

In the future, this size may change and may not be a power of two. Having
its own cache can also keep it from fragmenting memory.

Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Ryan Roberts <[email protected]>
Link: https://lore.kernel.org/[email protected]
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

kernel/trace/fgraph.c

@@ -172,6 +172,8 @@ enum {
 DEFINE_STATIC_KEY_FALSE(kill_ftrace_graph);
 int ftrace_graph_active;
 
+static struct kmem_cache *fgraph_stack_cachep;
+
 static struct fgraph_ops *fgraph_array[FGRAPH_ARRAY_SIZE];
 static unsigned long fgraph_array_bitmask;
 
@@ -1022,8 +1024,11 @@ static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
 	int start = 0, end = FTRACE_RETSTACK_ALLOC_SIZE;
 	struct task_struct *g, *t;
 
+	if (WARN_ON_ONCE(!fgraph_stack_cachep))
+		return -ENOMEM;
+
 	for (i = 0; i < FTRACE_RETSTACK_ALLOC_SIZE; i++) {
-		ret_stack_list[i] = kmalloc(SHADOW_STACK_SIZE, GFP_KERNEL);
+		ret_stack_list[i] = kmem_cache_alloc(fgraph_stack_cachep, GFP_KERNEL);
 		if (!ret_stack_list[i]) {
 			start = 0;
 			end = i;
@@ -1054,7 +1059,7 @@ static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
 	rcu_read_unlock();
 free:
 	for (i = start; i < end; i++)
-		kfree(ret_stack_list[i]);
+		kmem_cache_free(fgraph_stack_cachep, ret_stack_list[i]);
 	return ret;
 }
 
@@ -1117,9 +1122,12 @@ void ftrace_graph_init_idle_task(struct task_struct *t, int cpu)
 	if (ftrace_graph_active) {
 		unsigned long *ret_stack;
 
+		if (WARN_ON_ONCE(!fgraph_stack_cachep))
+			return;
+
 		ret_stack = per_cpu(idle_ret_stack, cpu);
 		if (!ret_stack) {
-			ret_stack = kmalloc(SHADOW_STACK_SIZE, GFP_KERNEL);
+			ret_stack = kmem_cache_alloc(fgraph_stack_cachep, GFP_KERNEL);
 			if (!ret_stack)
 				return;
 			per_cpu(idle_ret_stack, cpu) = ret_stack;
@@ -1139,7 +1147,10 @@ void ftrace_graph_init_task(struct task_struct *t)
 	if (ftrace_graph_active) {
 		unsigned long *ret_stack;
 
-		ret_stack = kmalloc(SHADOW_STACK_SIZE, GFP_KERNEL);
+		if (WARN_ON_ONCE(!fgraph_stack_cachep))
+			return;
+
+		ret_stack = kmem_cache_alloc(fgraph_stack_cachep, GFP_KERNEL);
 		if (!ret_stack)
 			return;
 		graph_init_task(t, ret_stack);
@@ -1154,7 +1165,11 @@ void ftrace_graph_exit_task(struct task_struct *t)
 	/* NULL must become visible to IRQs before we free it: */
 	barrier();
 
-	kfree(ret_stack);
+	if (ret_stack) {
+		if (WARN_ON_ONCE(!fgraph_stack_cachep))
+			return;
+		kmem_cache_free(fgraph_stack_cachep, ret_stack);
+	}
 }
 
 #ifdef CONFIG_DYNAMIC_FTRACE
@@ -1294,6 +1309,14 @@ int register_ftrace_graph(struct fgraph_ops *gops)
 
 	guard(mutex)(&ftrace_lock);
 
+	if (!fgraph_stack_cachep) {
+		fgraph_stack_cachep = kmem_cache_create("fgraph_stack",
+							SHADOW_STACK_SIZE,
+							SHADOW_STACK_SIZE, 0, NULL);
+		if (!fgraph_stack_cachep)
+			return -ENOMEM;
+	}
+
 	if (!fgraph_initialized) {
 		ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "fgraph:online",
 					fgraph_cpu_init, NULL);