v1.1: backport commitment max changes (#9775) (#9778)

automerge

ledger/src/bank_forks.rs

@@ -165,6 +165,7 @@ impl BankForks {
         &mut self,
         root: Slot,
         snapshot_package_sender: &Option<SnapshotPackageSender>,
+        largest_confirmed_root: Option<Slot>,
     ) {
         self.root = root;
         let set_root_start = Instant::now();
@@ -205,7 +206,7 @@ impl BankForks {
             }
         }
 
-        self.prune_non_root(root);
+        self.prune_non_root(root, largest_confirmed_root);
 
         inc_new_counter_info!(
             "bank-forks_set_root_ms",
@@ -276,10 +277,19 @@ impl BankForks {
         Ok(())
     }
 
-    fn prune_non_root(&mut self, root: Slot) {
+    fn prune_non_root(&mut self, root: Slot, largest_confirmed_root: Option<Slot>) {
         let descendants = self.descendants();
-        self.banks
-            .retain(|slot, _| slot == &root || descendants[&root].contains(slot));
+        self.banks.retain(|slot, _| {
+            *slot == root
+                || descendants[&root].contains(slot)
+                || (*slot < root
+                    && *slot >= largest_confirmed_root.unwrap_or(root)
+                    && descendants[slot].contains(&root))
+        });
+        datapoint_debug!(
+            "bank_forks_purge_non_root",
+            ("num_banks_retained", self.banks.len(), i64),
+        );
     }
 
     pub fn set_snapshot_config(&mut self, snapshot_config: Option<SnapshotConfig>) {