check-before-deployment: add selinux (#19703)

Author: dveeden

check-before-deployment.md

@@ -781,3 +781,7 @@ sudo yum -y install numactl
     ```
 
     To get help information of the `tiup cluster exec` command, run the `tiup cluster exec --help` command.
+
+## Disable SELinux
+
+Use the [getenforce(8)](https://linux.die.net/man/8/getenforce) utility to check if SELinux is disabled or set to permissive. SELinux in enforcing mode can cause deployment failures. For instructions on disabling SELinux, refer to your operating system's documentation.

dm/quick-start-with-dm.md

@@ -8,6 +8,10 @@ aliases: ['/docs/tidb-data-migration/dev/get-started/']
 
 This document describes how to migrate data from MySQL to TiDB using [TiDB Data Migration (DM)](/dm/dm-overview.md). This guide is a quick demo of DM features and is not recommended for any production environment.
 
+> **Note:**
+>
+> If your target machine's operating system supports SELinux, ensure that SELinux is **disabled**.
+
 ## Step 1: Deploy a DM cluster
 
 1. Install TiUP, and install [`dmctl`](/dm/dmctl-introduction.md) using TiUP: