[PATCH v2] tracing: Allocate mask_str buffer dynamically

[changbin . du]

From: Changbin Du <changbin.du@xxxxxxxxx>

The default NR_CPUS can be very large, but actual possible nr_cpu_ids
usually is very small. For my x86 distribution, the NR_CPUS is 8192 and
nr_cpu_ids is 4. About 2 pages are wasted.

Most machines don't have so many CPUs, so define a array with NR_CPUS
just wastes memory. So let's allocate the buffer dynamically when need.

The exact buffer size should be:
  DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;

Example output:
  ff,ffffffff

Signed-off-by: Changbin Du <changbin.du@xxxxxxxxx>

---
v2:
  - remove 'static' declaration.
  - fix buffer size.
---
 kernel/trace/trace.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 752e5da..6b70648 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4184,31 +4184,33 @@ static const struct file_operations show_traces_fops = {
  */
 static DEFINE_MUTEX(tracing_cpumask_update_lock);
 
-/*
- * Temporary storage for the character representation of the
- * CPU bitmask (and one more byte for the newline):
- */
-static char mask_str[NR_CPUS + 1];
-
 static ssize_t
 tracing_cpumask_read(struct file *filp, char __user *ubuf,
 		     size_t count, loff_t *ppos)
 {
 	struct trace_array *tr = file_inode(filp)->i_private;
+	char *mask_str;
 	int len;
 
+	/* Bitmap, ',' and two more bytes for the newline and '\0'. */
+	len = DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;
+	mask_str = kmalloc(len, GFP_KERNEL);
+	if (!mask_str)
+		return -ENOMEM;
+
 	mutex_lock(&tracing_cpumask_update_lock);
 
-	len = snprintf(mask_str, count, "%*pb\n",
+	len = snprintf(mask_str, len, "%*pb\n",
 		       cpumask_pr_args(tr->tracing_cpumask));
 	if (len >= count) {
 		count = -EINVAL;
 		goto out_err;
 	}
-	count = simple_read_from_buffer(ubuf, count, ppos, mask_str, NR_CPUS+1);
+	count = simple_read_from_buffer(ubuf, count, ppos, mask_str, len);
 
 out_err:
 	mutex_unlock(&tracing_cpumask_update_lock);
+	kfree(mask_str);
 
 	return count;
 }
-- 
2.7.4