[PATCH 3/9] mm/damon/dbgfs: implement deprecation notice file

[SeongJae Park <sj@kernel.org>]

Implement a read-only file for DAMON debugfs interface deprecation
notice, to let users who manually read/write the DAMON debugfs files
from their shell command line easily notice the fact.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/dbgfs.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index 7dac24e69e3b..fc6ece5a9f37 100644
--- a/mm/damon/dbgfs.c
+++ b/mm/damon/dbgfs.c
@@ -805,6 +805,18 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx)
 	damon_destroy_ctx(ctx);
 }
 
+static ssize_t damon_dbgfs_deprecated_read(struct file *file,
+		char __user *buf, size_t count, loff_t *ppos)
+{
+	char kbuf[512] = "DAMON debugfs interface is deprecated, "
+		     "so users should move to DAMON_SYSFS. If you cannot, "
+		     "please report your usecase to damon@lists.linux.dev and "
+		     "linux-mm@kvack.org.\n";
+	int len = strnlen(kbuf, 1024);
+
+	return simple_read_from_buffer(buf, count, ppos, kbuf, len);
+}
+
 /*
  * Make a context of @name and create a debugfs directory for it.
  *
@@ -1056,6 +1068,10 @@ static int damon_dbgfs_static_file_open(struct inode *inode, struct file *file)
 	return nonseekable_open(inode, file);
 }
 
+static const struct file_operations deprecated_fops = {
+	.read = damon_dbgfs_deprecated_read,
+};
+
 static const struct file_operations mk_contexts_fops = {
 	.open = damon_dbgfs_static_file_open,
 	.write = dbgfs_mk_context_write,
@@ -1076,9 +1092,9 @@ static int __init __damon_dbgfs_init(void)
 {
 	struct dentry *dbgfs_root;
 	const char * const file_names[] = {"mk_contexts", "rm_contexts",
-		"monitor_on"};
+		"monitor_on", "DEPRECATED"};
 	const struct file_operations *fops[] = {&mk_contexts_fops,
-		&rm_contexts_fops, &monitor_on_fops};
+		&rm_contexts_fops, &monitor_on_fops, &deprecated_fops};
 	int i;
 
 	dbgfs_root = debugfs_create_dir("damon", NULL);
-- 
2.39.2