From 0519138b04ae4b711b4a5b3328cc7e9495ca9962 Mon Sep 17 00:00:00 2001
From: Em Sharnoff <sharnoff@neon.tech>
Date: Wed, 10 Apr 2024 06:13:48 -0700
Subject: [PATCH] compute_ctl: Auto-set dynamic_shared_memory_type (#7348)

Part of neondatabase/cloud#12047.

The basic idea is that for our VMs, we want to enable swap and disable
Linux memory overcommit. Alongside these, we should set postgres'
dynamic_shared_memory_type to mmap, but we want to avoid setting it to
mmap if swap is not enabled.

Implementing this in the control plane would be fiddly, but it's
relatively straightforward to add to compute_ctl.
---
 compute_tools/src/config.rs     | 25 +++++++++++++++++++++++--
 compute_tools/src/pg_helpers.rs |  2 +-
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/compute_tools/src/config.rs b/compute_tools/src/config.rs
index f1fd8637f5..89c866b20c 100644
--- a/compute_tools/src/config.rs
+++ b/compute_tools/src/config.rs
@@ -6,8 +6,8 @@ use std::path::Path;
 use anyhow::Result;
 
 use crate::pg_helpers::escape_conf_value;
-use crate::pg_helpers::PgOptionsSerialize;
-use compute_api::spec::{ComputeMode, ComputeSpec};
+use crate::pg_helpers::{GenericOptionExt, PgOptionsSerialize};
+use compute_api::spec::{ComputeMode, ComputeSpec, GenericOption};
 
 /// Check that `line` is inside a text file and put it there if it is not.
 /// Create file if it doesn't exist.
@@ -92,6 +92,27 @@ pub fn write_postgres_conf(
         }
     }
 
+    if cfg!(target_os = "linux") {
+        // Check /proc/sys/vm/overcommit_memory -- if it equals 2 (i.e. linux memory overcommit is
+        // disabled), then the control plane has enabled swap and we should set
+        // dynamic_shared_memory_type = 'mmap'.
+        //
+        // This is (maybe?) temporary - for more, see https://github.com/neondatabase/cloud/issues/12047.
+        let overcommit_memory_contents = std::fs::read_to_string("/proc/sys/vm/overcommit_memory")
+            // ignore any errors - they may be expected to occur under certain situations (e.g. when
+            // not running in Linux).
+            .unwrap_or_else(|_| String::new());
+        if overcommit_memory_contents.trim() == "2" {
+            let opt = GenericOption {
+                name: "dynamic_shared_memory_type".to_owned(),
+                value: Some("mmap".to_owned()),
+                vartype: "enum".to_owned(),
+            };
+
+            write!(file, "{}", opt.to_pg_setting())?;
+        }
+    }
+
     // If there are any extra options in the 'settings' field, append those
     if spec.cluster.settings.is_some() {
         writeln!(file, "# Managed by compute_ctl: begin")?;
diff --git a/compute_tools/src/pg_helpers.rs b/compute_tools/src/pg_helpers.rs
index 5deb50d6b7..fa0822748b 100644
--- a/compute_tools/src/pg_helpers.rs
+++ b/compute_tools/src/pg_helpers.rs
@@ -44,7 +44,7 @@ pub fn escape_conf_value(s: &str) -> String {
     format!("'{}'", res)
 }
 
-trait GenericOptionExt {
+pub trait GenericOptionExt {
     fn to_pg_option(&self) -> String;
     fn to_pg_setting(&self) -> String;
 }