tracing: Resize tgid_map to pid_max, 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 the configured pid_max instead,
allowing it to cover the full range of PIDs up to the maximum value of
PID_MAX_LIMIT if the system is configured that way.

On 64 bit systems with pid_max == PID_MAX_LIMIT 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 could also increase for CONFIG_BASE_SMALL=y
configurations, but these seem unlikely to be systems upon which people
are both configuring a large pid_max and running ftrace with record-tgid
anyway.

Of note is that we only allocate tgid_map once, the first time that the
record-tgid option is enabled. Therefore its size is only set once, to
the value of pid_max at the time the record-tgid option is first
enabled. If a user increases pid_max after that point, the saved_tgids
file will not contain entries for any tasks with pids beyond the earlier
value of pid_max.

Link: https://lkml.kernel.org/r/20210701172407.889626-2-paulburton@google.com
Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks")
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Burton <paulburton@google.com>
[ Fixed comment coding style ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 4843076..14f56e9 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2191,8 +2191,15 @@ void tracing_reset_all_online_cpus(void)
        }
 }
 
+/*
+ * The tgid_map array maps from pid to tgid; i.e. the value stored at index i
+ * is the tgid last observed corresponding to pid=i.
+ */
 static int *tgid_map;
 
+/* The maximum valid index into tgid_map. */
+static size_t tgid_map_max;
+
 #define SAVED_CMDLINES_DEFAULT 128
 #define NO_CMDLINE_MAP UINT_MAX
 static arch_spinlock_t trace_cmdline_lock = __ARCH_SPIN_LOCK_UNLOCKED;
@@ -2468,24 +2475,41 @@ void trace_find_cmdline(int pid, char comm[])
        preempt_enable();
 }
 
+static int *trace_find_tgid_ptr(int pid)
+{
+       /*
+        * Pairs with the smp_store_release in set_tracer_flag() to ensure that
+        * if we observe a non-NULL tgid_map then we also observe the correct
+        * tgid_map_max.
+        */
+       int *map = smp_load_acquire(&tgid_map);
+
+       if (unlikely(!map || pid > tgid_map_max))
+               return NULL;
+
+       return &map[pid];
+}
+
 int trace_find_tgid(int pid)
 {
-       if (unlikely(!tgid_map || !pid || pid > PID_MAX_DEFAULT))
-               return 0;
+       int *ptr = trace_find_tgid_ptr(pid);
 
-       return tgid_map[pid];
+       return ptr ? *ptr : 0;
 }
 
 static int trace_save_tgid(struct task_struct *tsk)
 {
+       int *ptr;
+
        /* treat recording of idle task as a success */
        if (!tsk->pid)
                return 1;
 
-       if (unlikely(!tgid_map || tsk->pid > PID_MAX_DEFAULT))
+       ptr = trace_find_tgid_ptr(tsk->pid);
+       if (!ptr)
                return 0;
 
-       tgid_map[tsk->pid] = tsk->tgid;
+       *ptr = tsk->tgid;
        return 1;
 }
 
@@ -5225,6 +5249,8 @@ int trace_keep_overwrite(struct tracer *tracer, u32 mask, int set)
 
 int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled)
 {
+       int *map;
+
        if ((mask == TRACE_ITER_RECORD_TGID) ||
            (mask == TRACE_ITER_RECORD_CMD))
                lockdep_assert_held(&event_mutex);
@@ -5247,10 +5273,19 @@ int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled)
                trace_event_enable_cmd_record(enabled);
 
        if (mask == TRACE_ITER_RECORD_TGID) {
-               if (!tgid_map)
-                       tgid_map = kvcalloc(PID_MAX_DEFAULT + 1,
-                                          sizeof(*tgid_map),
-                                          GFP_KERNEL);
+               if (!tgid_map) {
+                       tgid_map_max = pid_max;
+                       map = kvcalloc(tgid_map_max + 1, sizeof(*tgid_map),
+                                      GFP_KERNEL);
+
+                       /*
+                        * Pairs with smp_load_acquire() in
+                        * trace_find_tgid_ptr() to ensure that if it observes
+                        * the tgid_map we just allocated then it also observes
+                        * the corresponding tgid_map_max value.
+                        */
+                       smp_store_release(&tgid_map, map);
+               }
                if (!tgid_map) {
                        tr->trace_flags &= ~TRACE_ITER_RECORD_TGID;
                        return -ENOMEM;
@@ -5664,18 +5699,14 @@ static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos)
 {
        int pid = ++(*pos);
 
-       if (pid > PID_MAX_DEFAULT)
-               return NULL;
-
-       return &tgid_map[pid];
+       return trace_find_tgid_ptr(pid);
 }
 
 static void *saved_tgids_start(struct seq_file *m, loff_t *pos)
 {
-       if (!tgid_map || *pos > PID_MAX_DEFAULT)
-               return NULL;
+       int pid = *pos;
 
-       return &tgid_map[*pos];
+       return trace_find_tgid_ptr(pid);
 }
 
 static void saved_tgids_stop(struct seq_file *m, void *v)