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

Masami Hiramatsu (Google)
2025-02-10T15:44:22Z

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;