[PATCH v4 1/3] tracing: Have saved_cmdlines arrays all in one allocation

[Steven Rostedt <rostedt@goodmis.org>]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The saved_cmdlines have three arrays for mapping PIDs to COMMs:

 - map_pid_to_cmdline[]
 - map_cmdline_to_pid[]
 - saved_cmdlines

The map_pid_to_cmdline[] is PID_MAX_DEFAULT in size and holds the index
into the other arrays. The map_cmdline_to_pid[] is a mapping back to the
full pid as it can be larger than PID_MAX_DEFAULT. And the
saved_cmdlines[] just holds the COMMs associated to the pids.

Currently the map_pid_to_cmdline[] and saved_cmdlines[] are allocated
together (in reality the saved_cmdlines is just in the memory of the
rounding of the allocation of the structure as it is always allocated in
powers of two). The map_cmdline_to_pid[] array is allocated separately.

Since the rounding to a power of two is rather large (it allows for 8000
elements in saved_cmdlines), also include the map_cmdline_to_pid[] array.
(This drops it to 6000 by default, which is still plenty for most use
cases). This saves even more memory as the map_cmdline_to_pid[] array
doesn't need to be allocated.

Link: https://lore.kernel.org/linux-trace-kernel/20240212174011.068211d9@gandalf.local.home/

Fixes: 44dc5c41b5b1 ("tracing: Fix wasted memory in saved_cmdlines logic")
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v4: https://lore.kernel.org/linux-trace-kernel/20240216210150.379809261@goodmis.org

- Do not kfree() map_cmdline_to_pid now that it is included in the
  page allocations.

 kernel/trace/trace.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8198bfc54b58..52faa30e64ed 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2325,6 +2325,10 @@ struct saved_cmdlines_buffer {
 };
 static struct saved_cmdlines_buffer *savedcmd;
 
+/* Holds the size of a cmdline and pid element */
+#define SAVED_CMDLINE_MAP_ELEMENT_SIZE(s)			\
+	(TASK_COMM_LEN + sizeof((s)->map_cmdline_to_pid[0]))
+
 static inline char *get_saved_cmdlines(int idx)
 {
 	return &savedcmd->saved_cmdlines[idx * TASK_COMM_LEN];
@@ -2339,7 +2343,6 @@ static void free_saved_cmdlines_buffer(struct saved_cmdlines_buffer *s)
 {
 	int order = get_order(sizeof(*s) + s->cmdline_num * TASK_COMM_LEN);
 
-	kfree(s->map_cmdline_to_pid);
 	kmemleak_free(s);
 	free_pages((unsigned long)s, order);
 }
@@ -2352,7 +2355,7 @@ static struct saved_cmdlines_buffer *allocate_cmdlines_buffer(unsigned int val)
 	int order;
 
 	/* Figure out how much is needed to hold the given number of cmdlines */
-	orig_size = sizeof(*s) + val * TASK_COMM_LEN;
+	orig_size = sizeof(*s) + val * SAVED_CMDLINE_MAP_ELEMENT_SIZE(s);
 	order = get_order(orig_size);
 	size = 1 << (order + PAGE_SHIFT);
 	page = alloc_pages(GFP_KERNEL, order);
@@ -2364,16 +2367,11 @@ static struct saved_cmdlines_buffer *allocate_cmdlines_buffer(unsigned int val)
 	memset(s, 0, sizeof(*s));
 
 	/* Round up to actual allocation */
-	val = (size - sizeof(*s)) / TASK_COMM_LEN;
+	val = (size - sizeof(*s)) / SAVED_CMDLINE_MAP_ELEMENT_SIZE(s);
 	s->cmdline_num = val;
 
-	s->map_cmdline_to_pid = kmalloc_array(val,
-					      sizeof(*s->map_cmdline_to_pid),
-					      GFP_KERNEL);
-	if (!s->map_cmdline_to_pid) {
-		free_saved_cmdlines_buffer(s);
-		return NULL;
-	}
+	/* Place map_cmdline_to_pid array right after saved_cmdlines */
+	s->map_cmdline_to_pid = (unsigned *)&s->saved_cmdlines[val * TASK_COMM_LEN];
 
 	s->cmdline_idx = 0;
 	memset(&s->map_pid_to_cmdline, NO_CMDLINE_MAP,
-- 
2.43.0