linux-kernel - [PATCH 2/2] tracing: Resize tgid_map to PID_MAX_LIMIT, not PID_MAX

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Date:   Tue, 29 Jun 2021 17:34:06 -0700
From:   Paul Burton <paulburton@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Paul Burton <paulburton@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>, stable@...r.kernel.org
Subject: [PATCH 2/2] tracing: Resize tgid_map to PID_MAX_LIMIT, not PID_MAX_DEFAULT

Currently tgid_map is sized at PID_MAX_DEFAULT entries, which means that
on systems where pid_max is configured higher than PID_MAX_DEFAULT the
ftrace record-tgid option doesn't work so well. Any tasks with PIDs
higher than PID_MAX_DEFAULT are simply not recorded in tgid_map, and
don't show up in the saved_tgids file.

In particular since systemd v243 & above configure pid_max to its
highest possible 1<<22 value by default on 64 bit systems this renders
the record-tgids option of little use.

Increase the size of tgid_map to PID_MAX_LIMIT instead, allowing it to
cover the full range of PIDs up to the maximum value of pid_max.

On 64 bit systems this will increase the size of tgid_map from 256KiB to
16MiB. Whilst this 64x increase in memory overhead sounds significant 64
bit systems are presumably best placed to accommodate it, and since
tgid_map is only allocated when the record-tgid option is actually used
presumably the user would rather it spends sufficient memory to actually
record the tgids they expect.

The size of tgid_map will also increase for CONFIG_BASE_SMALL=y
configurations, but these seem unlikely to be systems upon which people
are running ftrace with record-tgid anyway.

Signed-off-by: Paul Burton <paulburton@...gle.com>
Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks")
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Joel Fernandes <joelaf@...gle.com>
Cc: <stable@...r.kernel.org>
---
 kernel/trace/trace.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 9570667310bcc..c893c0c2754f8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2460,7 +2460,7 @@ void trace_find_cmdline(int pid, char comm[])

 int trace_find_tgid(int pid)
 {
-	if (unlikely(!tgid_map || !pid || pid > PID_MAX_DEFAULT))
+	if (unlikely(!tgid_map || !pid || pid > PID_MAX_LIMIT))
 		return 0;

 	return tgid_map[pid];
@@ -2472,7 +2472,7 @@ static int trace_save_tgid(struct task_struct *tsk)
 	if (!tsk->pid)
 		return 1;

-	if (unlikely(!tgid_map || tsk->pid > PID_MAX_DEFAULT))
+	if (unlikely(!tgid_map || tsk->pid > PID_MAX_LIMIT))
 		return 0;

 	tgid_map[tsk->pid] = tsk->tgid;
@@ -5194,7 +5194,7 @@ int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled)

 	if (mask == TRACE_ITER_RECORD_TGID) {
 		if (!tgid_map)
-			tgid_map = kvcalloc(PID_MAX_DEFAULT + 1,
+			tgid_map = kvcalloc(PID_MAX_LIMIT + 1,
 					   sizeof(*tgid_map),
 					   GFP_KERNEL);
 		if (!tgid_map) {
@@ -5610,7 +5610,7 @@ static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos)
 {
 	int pid = ++(*pos);

-	if (pid > PID_MAX_DEFAULT)
+	if (pid > PID_MAX_LIMIT)
 		return NULL;

 	return &tgid_map[pid];
@@ -5618,7 +5618,7 @@ static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos)

 static void *saved_tgids_start(struct seq_file *m, loff_t *pos)
 {
-	if (!tgid_map || *pos > PID_MAX_DEFAULT)
+	if (!tgid_map || *pos > PID_MAX_LIMIT)
 		return NULL;

 	return &tgid_map[*pos];
-- 
2.32.0.93.g670b81a890-goog