[PATCH v5 3/3] rust_binder: shrink all_procs when deregistering processes

[Shivam Kalra via B4 Relay]

From: Shivam Kalra <shivamkalra98@xxxxxxxxxxx>

When a process is deregistered from the binder context, the all_procs
vector may have significant unused capacity. Add logic to shrink the
vector using a conservative strategy that prevents shrink-then-regrow
oscillation.

The shrinking strategy triggers when length drops below 1/4 of capacity,
and shrinks to twice the current length rather than to the exact length.
This provides hysteresis to avoid repeated reallocations when the process
count fluctuates.

The shrink operation uses GFP_KERNEL and is allowed to fail gracefully
since it is purely an optimization. The vector remains valid and
functional even if shrinking fails.

Suggested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Signed-off-by: Shivam Kalra <shivamkalra98@xxxxxxxxxxx>
---
 drivers/android/binder/context.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/android/binder/context.rs b/drivers/android/binder/context.rs
index 9cf437c025a20..ddddb66b35571 100644
--- a/drivers/android/binder/context.rs
+++ b/drivers/android/binder/context.rs
@@ -94,6 +94,17 @@ pub(crate) fn deregister_process(self: &Arc<Self>, proc: &Arc<Process>) {
         }
         let mut manager = self.manager.lock();
         manager.all_procs.retain(|p| !Arc::ptr_eq(p, proc));
+
+        // Shrink the vector if it has significant unused capacity to avoid memory waste,
+        // but use a conservative strategy to prevent shrink-then-regrow oscillation.
+        // Only shrink when length drops below 1/4 of capacity, and shrink to twice the length.
+        let len = manager.all_procs.len();
+        let cap = manager.all_procs.capacity();
+        if len < cap / 4 {
+            // Shrink to twice the current length. Ignore allocation failures since this
+            // is just an optimization; the vector remains valid even if shrinking fails.
+            let _ = manager.all_procs.shrink_to(len * 2, GFP_KERNEL);
+        }
     }
 
     pub(crate) fn set_manager_node(&self, node_ref: NodeRef) -> Result {

-- 
2.43.0