[RFC PATCH v2 3/3] tracing: Freeable reserved ring buffer

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Make the ring buffer on reserved memory to be freeable. This allows us
to free the trace instance on the reserved memory without changing
cmdline and rebooting. Even if we can not change the kernel cmdline
for security reason, we can release the reserved memory for the ring
buffer as free (available) memory.

For example, boot kernel with reserved memory;
"reserve_mem=20M:2M:trace trace_instance=boot_mapped^traceoff@trace"

 # free
              total        used        free      shared  buff/cache   available
Mem:        1994720       45292     1931960       14908       17468     1915920
Swap:             0           0           0
 # rmdir /sys/kernel/tracing/instances/boot_mapped/
 # free
              total        used        free      shared  buff/cache   available
Mem:        1994720       17204     1960060       14912       17456     1944012
Swap:             0           0           0

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/trace/trace.c |   17 +++++++++++++++--
 kernel/trace/trace.h |    1 +
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index dde8000a5614..113ad3179ecd 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9594,6 +9594,9 @@ static void free_trace_buffers(struct trace_array *tr)
 #ifdef CONFIG_TRACER_MAX_TRACE
 	free_trace_buffer(&tr->max_buffer);
 #endif
+
+	if (tr->range_addr_start)
+		vunmap((void *)tr->range_addr_start);
 }
 
 static void init_trace_flags_index(struct trace_array *tr)
@@ -9755,6 +9758,7 @@ trace_array_create_systems(const char *name, const char *systems,
 	free_cpumask_var(tr->pipe_cpumask);
 	free_cpumask_var(tr->tracing_cpumask);
 	kfree_const(tr->system_names);
+	kfree(tr->range_name);
 	kfree(tr->name);
 	kfree(tr);
 
@@ -9857,8 +9861,10 @@ static int __remove_instance(struct trace_array *tr)
 	int i;
 
 	/* Reference counter for a newly created trace array = 1. */
-	if (tr->ref > 1 || (tr->current_trace && tr->trace_ref))
+	if (tr->ref > 1 || (tr->current_trace && tr->trace_ref)) {
+		pr_info("Instance is busy: ref %d trace_ref %d\n", tr->ref, tr->trace_ref);
 		return -EBUSY;
+	}
 
 	list_del(&tr->list);
 
@@ -9881,6 +9887,11 @@ static int __remove_instance(struct trace_array *tr)
 	free_trace_buffers(tr);
 	clear_tracing_err_log(tr);
 
+	if (tr->range_name) {
+		reserved_mem_release_by_name(tr->range_name);
+		kfree(tr->range_name);
+	}
+
 	for (i = 0; i < tr->nr_topts; i++) {
 		kfree(tr->topts[i].topts);
 	}
@@ -10740,6 +10751,7 @@ __init static void enable_instances(void)
 		bool traceoff = false;
 		char *flag_delim;
 		char *addr_delim;
+		char *rname __free(kfree) = NULL;
 
 		tok = strsep(&curr_str, ",");
 
@@ -10796,6 +10808,7 @@ __init static void enable_instances(void)
 				pr_warn("Failed to map boot instance %s to %s\n", name, tok);
 				continue;
 			}
+			rname = kstrdup(tok, GFP_KERNEL);
 		}
 
 		if (start) {
@@ -10832,7 +10845,7 @@ __init static void enable_instances(void)
 		 */
 		if (start) {
 			tr->flags |= TRACE_ARRAY_FL_BOOT | TRACE_ARRAY_FL_LAST_BOOT;
-			tr->ref++;
+			tr->range_name = no_free_ptr(rname);
 		}
 
 		while ((tok = strsep(&curr_str, ","))) {
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 47c0742fe9ec..ae8f7fac6592 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -348,6 +348,7 @@ struct trace_array {
 	unsigned int		mapped;
 	unsigned long		range_addr_start;
 	unsigned long		range_addr_size;
+	char			*range_name;
 	long			text_delta;
 	int			nr_modules;
 	long			*module_delta;