Linux Memory Region Data Structures

The memory descriptor, known as mm_struct, holds crucial data about a process's address space, including the amount of memory it occupies. Within this structure, there is a linked list of Virtual Memory Areas (VMAs) represented by vm_area_struct. This list is accessed via the mmap field of mm_struct, which points to the first memory region in the linked list.

Each VMA contains a pointer to the subsequent memory region (vm_next), enabling the kernel to traverse all virtual memory regions belonging to the process. The total number of VMAs owned by the process is recorded in the map_count field. The Linux kernel sets a default maximum of 65,536 VMAs, although most processes utilize far fewer.

When a process frequently allocates memory—through methods like malloc(), shared libraries, or memory-mapped files—it may accumulate a large number of VMAs, ranging from hundreds to thousands. Traversing the linked list to locate a specific memory region requires examining each node sequentially, resulting in a time complexity of O(n).

To expedite lookup processes, Linux employs a red-black tree, an advanced data structure that maintains balance. This tree is embedded within each VMA as a vm_rb field of type struct rb_node. The root of the tree is stored in the process's memory descriptor (mm_rb). The tree is organized by the starting virtual address (vm_start) of each memory region, with smaller addresses placed in the left subtree and larger addresses in the right subtree.

The red-black tree upholds balance through several properties: nodes are colored either red or black, the root node is always black, red nodes cannot have consecutive red children, and every path from a node to a null descendant leaf contains the same number of black nodes. This ensures the tree's height remains at most 2 log₂(n + 1), allowing for efficient searching, inserting, and deleting of VMAs in O(log n) time.

In recent Linux versions (starting from 6.1), the kernel has replaced both the linked list (mmap) and red-black tree (mm_rb) for managing VMAs with a single data structure called the Maple Tree. The mm_struct now includes a single field, struct maple_tree mm_mt. The Maple Tree addresses the limitations of its predecessors by storing multiple memory regions within each node, akin to a B-tree.

This allows for efficient searching, insertion, and deletion of VMAs while maintaining a time complexity of O(log n). However, the Maple Tree offers lower constant overhead and superior real-world performance compared to the previous implementation.