vm-monitor: Don't set cgroup memory.max (#5333)

All it does is make postgres OOM more often (which, tbf, means we're
less likely to have e.g. compute_ctl get OOM-killed, but that tradeoff
isn't worth it).

Internally, this means removing all references to `memory.max` and the
places where we calculate or store the intended value.

As discussed in the sync earlier.

ref:

- https://neondb.slack.com/archives/C03H1K0PGKH/p1694698949252439?thread_ts=1694505575.693449&cid=C03H1K0PGKH
- https://neondb.slack.com/archives/C03H1K0PGKH/p1695049198622759

libs/vm_monitor/src/cgroup.rs

@@ -561,14 +561,7 @@ impl CgroupWatcher {
 /// Setting these values also affects the thresholds for receiving usage alerts.
 #[derive(Debug)]
 pub struct MemoryLimits {
-    high: u64,
-    max: u64,
-}
-
-impl MemoryLimits {
-    pub fn new(high: u64, max: u64) -> Self {
-        Self { max, high }
-    }
+    pub high: u64,
 }
 
 // Methods for manipulating the actual cgroup
@@ -645,12 +638,7 @@ impl CgroupWatcher {
 
     /// Set cgroup memory.high and memory.max.
     pub fn set_limits(&self, limits: &MemoryLimits) -> anyhow::Result<()> {
-        info!(
-            limits.high,
-            limits.max,
-            path = self.path(),
-            "writing new memory limits",
-        );
+        info!(limits.high, path = self.path(), "writing new memory limits",);
         self.memory()
             .context("failed to get memory subsystem while setting memory limits")?
             .set_mem(cgroups_rs::memory::SetMemory {
@@ -659,7 +647,7 @@ impl CgroupWatcher {
                 high: Some(MaxValue::Value(
                     u64::min(limits.high, i64::MAX as u64) as i64
                 )),
-                max: Some(MaxValue::Value(u64::min(limits.max, i64::MAX as u64) as i64)),
+                max: None,
             })
             .context("failed to set memory limits")
     }
@@ -667,7 +655,7 @@ impl CgroupWatcher {
     /// Given some amount of available memory, set the desired cgroup memory limits
     pub fn set_memory_limits(&mut self, available_memory: u64) -> anyhow::Result<()> {
         let new_high = self.config.calculate_memory_high_value(available_memory);
-        let limits = MemoryLimits::new(new_high, available_memory);
+        let limits = MemoryLimits { high: new_high };
         info!(
             path = self.path(),
             memory = ?limits,

libs/vm_monitor/src/runner.rs

@@ -257,12 +257,11 @@ impl Runner {
                 new_cgroup_mem_high = cgroup.config.calculate_memory_high_value(available_memory);
             }
 
-            let limits = MemoryLimits::new(
+            let limits = MemoryLimits {
                 // new_cgroup_mem_high is initialized to 0 but it is guarancontextd to not be here
                 // since it is properly initialized in the previous cgroup if let block
-                new_cgroup_mem_high,
-                available_memory,
-            );
+                high: new_cgroup_mem_high,
+            };
             cgroup
                 .set_limits(&limits)
                 .context("failed to set cgroup memory limits")?;
@@ -328,7 +327,9 @@ impl Runner {
                 name = cgroup.path(),
                 "updating cgroup memory.high",
             );
-            let limits = MemoryLimits::new(new_cgroup_mem_high, available_memory);
+            let limits = MemoryLimits {
+                high: new_cgroup_mem_high,
+            };
             cgroup
                 .set_limits(&limits)
                 .context("failed to set file cache size")?;