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This article explains the Linux kernel buddy system, detailing its data structures, initialization, allocation and free processes, and discusses its advantages and drawbacks in managing physical memory and reducing fragmentation.
Linux’s Contiguous Memory Allocator (CMA) reserves and manages large physical memory blocks for devices by integrating with the Buddy system, using structures like struct cma, bitmap allocation, and APIs such as cma_init_reserved_mem and cma_alloc, while handling page migration, LRU and per‑CPU PCP caches to ensure efficient allocation and release.
This article explains the Linux kernel memory management hierarchy—including NUMA nodes, memory zones, the buddy system for free pages, and the SLAB allocator—providing command‑line examples, code snippets, and visual diagrams to illustrate how the kernel efficiently allocates and reclaims memory.
Linux uses a buddy‑system allocator organized into nodes and zones to manage physical memory, distinguishing internal fragmentation (unused space within allocated blocks) from external fragmentation (insufficient contiguous free blocks), and mitigates the latter through memory compaction, the kcompactd daemon, and various allocation‑policy optimizations.