rust_binder: shrink all_procs when deregistering processes

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@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260216-binder-shrink-vec-v3-v6-3-ece8e8593e53@zohomail.in
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder/context.rs

@@ -94,6 +94,17 @@ impl Context {
         }
         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 {