AUTHOR: @Marko Budiselic DATE: January 25, 2025

Intro

Virtual Memory (VM) mapping under Linux refers to how the kernel translates a process’s virtual address space into physical memory locations (or swap). Each process sees its own continuous range of addresses, while the kernel manages the behind-the-scenes mapping via page tables. When a process tries to access a virtual address, the system consults these page tables to find the corresponding physical page. If it isn’t present in RAM, the kernel may load it from disk (demand paging) or swap. This abstraction allows efficient use of memory and protects processes from interfering with each other’s data. Various regions (code, heap, stack, shared libraries) are mapped with appropriate permissions to ensure security and stability.

vm.max_map_count is a Linux kernel parameter that sets the maximum number of VM mappings a process can have. Each mapping represents a specific range in a process’s virtual address space, used for loading files, shared libraries, memory-mapped I/O, or anonymous memory allocations. For example, each dynamic library or separate file mapping typically creates a new entry. If the number of mappings exceeds vm.max_map_count, further attempts to create new mappings will fail, potentially causing errors or crashes. This limit helps protect the system by preventing a single process from exhausting system resources with excessive mappings. However, some applications (especially those with many small mappings) may require a higher limit, so administrators sometimes adjust vm.max_map_count accordingly. Careful tuning is important—raising it too high may consume additional kernel memory for managing page tables, while keeping it too low can lead to application failures.

sysctl_max_map_count is the kernel override for the limit, you can see it used here:

• https://github.com/torvalds/linux/blob/v6.12/include/linux/mm.h#L206
• https://github.com/torvalds/linux/blob/v6.12/mm/mmap.c#L324-L325

vm.max_map_count does not change the underlying data structure, just how large it can get. What is being tracked is vm_area_struct, either new area, or grow/extend/split/remove an existing one. These are held in either a red black tree (older kernel), or a maple tree (newer kernel). In either case the actual memory consumption of these data structures does not matter, the memory consumed is only for what the program requires. It does not select different data structures or do alternative preallocation. Its just how many entries are in the tree and hence impacts how deep that tree can get (which can impact perf). FYI a newer kernel 6.1+, those maple trees use less memory, are not as deep, and cache friendly (advise running on linux 6.1 or newer).

The default value for vm.max_map_count is often sufficient for most use cases. However, workloads like databases or applications requiring numerous memory mappings (e.g., many files or shared libraries) might require higher limits.

Implications of vm.max_map_count Changes

• Increased Memory Usage - Each memory map (VM mapping) consumes some kernel memory. If you increase the vm.max_map_count, applications can create more memory mappings, which can lead to higher memory consumption in the kernel space. This can lead to reduced available memory for other processes and may cause performance degradation if not carefully managed.
• Kernel Overhead - A higher number of memory mappings means the kernel needs to manage more VMAs (Virtual Memory Areas). This can increase the overhead for certain kernel operations, such as managing page tables, which may result in slight performance degradation, especially under high loads.
• Potential for System Instability - Misconfigured or buggy applications might exploit the increased limit to create excessive mappings, potentially exhausting system resources (e.g., memory or file descriptors) and causing system instability or crashes.
• Security Concerns - Allowing more mappings can increase the attack surface for certain types of vulnerabilities. For instance, malicious users or processes might leverage the increased limit for resource exhaustion attacks or trigger bugs in kernel memory management.
• Unintended Behavior in Applications - Applications that rely on specific values of vm.max_map_count for regular operation might behave unpredictably if the limit is set too high or too low. This can lead to unexpected application crashes or resource leaks.
• Impact on Performance - Raising max_map_count does not impact the performance or operation of any process that would have been fine with the default of 65535.
• Impact on Performance Monitoring - Monitoring tools or processes that expect a certain number of mappings might find it challenging to operate effectively if the number of mappings increases significantly. This can complicate system management.
• Limited by Physical Resources - Simply increasing the limit does not guarantee better application performance or scalability. The actual limit might still be constrained by available physical memory and CPU capacity. If these are insufficient, setting a high vm.max_map_count could lead to swapping, thrashing, or overall system performance degradation.
• Compatibility Issues - Some older or poorly designed software might not handle a higher number of mappings well, causing compatibility issues.