Merge pull request #25638 from bluca/rate_limit_config

pid1: add env var to override default mount rate limit burst

docs/ENVIRONMENT.md

@@ -283,6 +283,13 @@ All tools:
   type as unsupported may not prevent loading some units of that type if they
   are referenced by other units of another supported type.
 
+* `$SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_BURST` — can be set to override the mount
+  units burst rate limit for parsing `/proc/self/mountinfo`. On a system with
+  few resources but many mounts the rate limit may be hit, which will cause the
+  processing of mount units to stall. The burst limit may be adjusted when the
+  default is not appropriate for a given system. Defaults to `5`, accepts
+  positive integers.
+
 `systemd-remount-fs`:
 
 * `$SYSTEMD_REMOUNT_ROOT_RW=1` — if set and no entry for the root directory