[PATCH v24 09/14] mm/damon/dbgfs: Export kdamond pid to the user space

[SeongJae Park <sjpark@amazon.com>]

From: SeongJae Park <sjpark@amazon.de>

For CPU usage accounting, knowing pid of the monitoring thread could be
helpful.  For example, users could use cpuaccount cgroups with the pid.

This commit therefore exports the pid of currently running monitoring
thread to the user space via 'kdamond_pid' file in the debugfs
directory.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 mm/damon/dbgfs.c | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index dce4409e5887..4b9ac2043e99 100644
--- a/mm/damon/dbgfs.c
+++ b/mm/damon/dbgfs.c
@@ -358,6 +358,32 @@ static ssize_t dbgfs_target_ids_write(struct file *file,
 	return ret;
 }
 
+static ssize_t dbgfs_kdamond_pid_read(struct file *file,
+		char __user *buf, size_t count, loff_t *ppos)
+{
+	struct damon_ctx *ctx = file->private_data;
+	char *kbuf;
+	ssize_t len;
+
+	kbuf = kmalloc(count, GFP_KERNEL);
+	if (!kbuf)
+		return -ENOMEM;
+
+	mutex_lock(&ctx->kdamond_lock);
+	if (ctx->kdamond)
+		len = scnprintf(kbuf, count, "%d\n", ctx->kdamond->pid);
+	else
+		len = scnprintf(kbuf, count, "none\n");
+	mutex_unlock(&ctx->kdamond_lock);
+	if (!len)
+		goto out;
+	len = simple_read_from_buffer(buf, count, ppos, kbuf, len);
+
+out:
+	kfree(kbuf);
+	return len;
+}
+
 static int damon_dbgfs_open(struct inode *inode, struct file *file)
 {
 	file->private_data = inode->i_private;
@@ -386,11 +412,18 @@ static const struct file_operations target_ids_fops = {
 	.write = dbgfs_target_ids_write,
 };
 
+static const struct file_operations kdamond_pid_fops = {
+	.owner = THIS_MODULE,
+	.open = damon_dbgfs_open,
+	.read = dbgfs_kdamond_pid_read,
+};
+
 static int dbgfs_fill_ctx_dir(struct dentry *dir, struct damon_ctx *ctx)
 {
-	const char * const file_names[] = {"attrs", "record", "target_ids"};
+	const char * const file_names[] = {"attrs", "record", "target_ids",
+		"kdamond_pid"};
 	const struct file_operations *fops[] = {&attrs_fops, &record_fops,
-		&target_ids_fops};
+		&target_ids_fops, &kdamond_pid_fops};
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(file_names); i++) {
-- 
2.17.1