From c55426445a732516c95d0ae7c0b6b77f06bb13d0 Mon Sep 17 00:00:00 2001
From: Luca Boccassi <bluca@debian.org>
Date: Mon, 5 Dec 2022 21:05:54 +0000
Subject: [PATCH] pid1: add env var to override default mount rate limit burst

I am hitting the rate limit on a busy system with low resources, and
it stalls the boot process which is Very Bad (TM).

(cherry picked from commit 24a4542cfa674ee80b54afcc223f2490a011966b)

Included in the backport because it's a workaround for an issue introduced
in v249.
---
 docs/ENVIRONMENT.md |  7 +++++++
 src/core/mount.c    | 11 ++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/docs/ENVIRONMENT.md b/docs/ENVIRONMENT.md
index ab3add6031..1e7a75a36c 100644
--- a/docs/ENVIRONMENT.md
+++ b/docs/ENVIRONMENT.md
@@ -268,6 +268,13 @@ All tools:
   it is either set to `system` or `user` depending on whether the NSS/PAM
   module is called by systemd in `--system` or `--user` mode.
 
+* `$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
diff --git a/src/core/mount.c b/src/core/mount.c
index 5e8a6ead61..dea7cd9d3e 100644
--- a/src/core/mount.c
+++ b/src/core/mount.c
@@ -1908,6 +1908,7 @@ static void mount_enumerate(Manager *m) {
         mnt_init_debug(0);
 
         if (!m->mount_monitor) {
+                unsigned mount_rate_limit_burst = 5;
                 int fd;
 
                 m->mount_monitor = mnt_new_monitor();
@@ -1947,7 +1948,15 @@ static void mount_enumerate(Manager *m) {
                         goto fail;
                 }
 
-                r = sd_event_source_set_ratelimit(m->mount_event_source, 1 * USEC_PER_SEC, 5);
+                /* Let users override the default (5 in 1s), as it stalls the boot sequence on busy systems. */
+                const char *e = secure_getenv("SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_BURST");
+                if (e) {
+                        r = safe_atou(e, &mount_rate_limit_burst);
+                        if (r < 0)
+                                log_debug("Invalid value in $SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_BURST, ignoring: %s", e);
+                }
+
+                r = sd_event_source_set_ratelimit(m->mount_event_source, 1 * USEC_PER_SEC, mount_rate_limit_burst);
                 if (r < 0) {
                         log_error_errno(r, "Failed to enable rate limit for mount events: %m");
                         goto fail;