[$] Memory controller performance improvements

The kernel's memory controller works within the control-group mechanism to
enforce memory-usage limits on groups of processes. This component has
often had performance problems, so there is continual interest in
optimizing it. Shakeel Butt led a session during the memory-management
track of the 2025 Linux Storage, Filesystem, Memory-Management, and BPF
Summit to look at the current state of the memory controller and what can
be done to reduce its overhead.