This series adds shrink_to() and shrink_to_fit() methods to Vec<T, A>
(and by extension KVVec, KVec, VVec) to allow explicit capacity
reduction for memory reclamation.

Problem:
When elements are removed from a KVVec, the allocated capacity is not
reduced. The underlying C allocators (krealloc/kvrealloc) don't shrink
memory in-place. This can cause significant memory waste in scenarios
with variable workloads.

Solution:
- Patch 1: Introduces the `Shrinkable` marker trait to identify allocators
  that can meaningfully reclaim memory when shrinking (e.g., Vmalloc but
  not Kmalloc, since Kmalloc uses fixed-size buckets).
- Patch 2: Implements shrink_to(min_capacity, flags) and
  shrink_to_fit(flags)  methods. Shrinking only occurs when
  the allocator is shrinkable AND the operation would free at least
  one page of memory.
- Patch 3: Adds KUnit tests for the new methods across different allocator
  backends (Vmalloc and KVmalloc).
- Patch 4: Uses shrink_to() in the Rust binder driver to reclaim memory
  when processes are deregistered. Shrinking is triggered only when the
  vector capacity exceeds 128 elements and less than half is used.

Testing:
- KUnit tests pass (rust_kvec test suite).
- Kernel boots successfully in QEMU with CONFIG_ANDROID_BINDER_IPC_RUST=y.

Changes since v2:
- Introduced new `Shrinkable` marker trait to distinguish allocators that
  benefit from shrinking (Vmalloc) from those that don't (Kmalloc).
- Shrinking now only occurs for vmalloc-backed buffers when at least one
  page can be freed, avoiding unnecessary work for kmalloc buffers.
  (Suggested by Danilo Krummrich)
- Split into 4 patches: Shrinkable trait introduction is now a
  separate patch to improve reviewability.
- Uses explicit alloc+copy+free since vrealloc doesn't yet support
  in-place shrinking; TODO added for future optimization when vrealloc
  gains that capability. (Discussed with Danilo Krummrich)
- Fixed minor KVVec/KVec terminology (binder uses KVVec not KVec).
- Expanded KUnit tests to cover different allocator backends and
  page-boundary shrinking behavior.

Changes since v1:
- Resend with correct threading (no code changes).
- Removed base-commit.
- Added explicit lore link to dependency in cover letter.

[1] https://lore.kernel.org/lkml/20260130205424.261700-1-shivamklr@cock.li/
[2] https://lore.kernel.org/lkml/20260131154016.270385-1-shivamklr@cock.li/

Signed-off-by: Shivam Kalra <shivamklr@cock.li>
---
Shivam Kalra (4):
      rust: alloc: introduce Shrinkable trait
      rust: kvec: implement shrink_to and shrink_to_fit for Vec
      rust: alloc: add KUnit tests for Vec shrink operations
      rust: binder: shrink all_procs when deregistering processes

 drivers/android/binder/context.rs |  10 ++
 rust/kernel/alloc/allocator.rs    |  48 +++++++
 rust/kernel/alloc/kvec.rs         | 296 +++++++++++++++++++++++++++++++++++++-
 3 files changed, 352 insertions(+), 2 deletions(-)
---
base-commit: 3c4ae63073d84abee5d81ce46d86a94e9dae9c89
change-id: 20260207-binder-shrink-vec-v3-54045d77b0e7

Best regards,
-- 
Shivam Kalra <shivamklr@cock.li>