mm/damon/dbgfs: implement deprecation notice file

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.

[arnd@arndb.de: fix bogus string length]
  Link: https://lkml.kernel.org/r/20240202124339.892862-1-arnd@kernel.org
Link: https://lkml.kernel.org/r/20240130013549.89538-4-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alex Shi <alexs@kernel.org>
Cc: Hu Haowen <2023002089@link.tyut.edu.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/dbgfs.c

@@ -805,6 +805,17 @@ 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)
+{
+	static const 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";
+
+	return simple_read_from_buffer(buf, count, ppos, kbuf, strlen(kbuf));
+}
+
 /*
  * Make a context of @name and create a debugfs directory for it.
  *
@@ -1056,6 +1067,10 @@ 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 +1091,9 @@ 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);