Does memory mapping segment and heap grow until they meet each other?

This question can come to anyone’s mind who is trying to understand the memory structure.

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Credits

RLIMIT_AS

  • Purpose: RLIMIT_AS sets the maximum size of the process’s virtual address space in bytes.
    • Scope: This limit applies to all types of memory allocation within the process, including:
      • The stack.
      • The heap (via brk).
      • Memory mapped regions (mmap).
      • Shared libraries.
    • Behavior: If an allocation would exceed this limit, it fails with an error or might result in the termination of the process.

RLIMIT_DATA

  • Purpose: RLIMIT_DATA restricts the maximum size of the data segment.
    • Components: This segment includes:
      • Initialized data (data section).
      • Uninitialized data (bss section).
      • Heap memory managed by brk and sbrk.
    • Expansion: Since Linux 4.7, this limit also affects mmap operations, influencing how much memory can be allocated for data.
    • Error Handling: If these limits are exceeded, operations like brk() will fail with ENOMEM.

Kernel’s Role in Memory Management

  • Limit Checks: The kernel continuously monitors:
    • Stack expansion.
    • mmap calls.
    • brk system calls (heap allocation).
    • Conflict Prevention: The kernel ensures that different memory segments do not overlap or conflict:
      • Checks are performed during heap allocation to prevent overwriting existing segments.
      • If a conflict is detected, the allocation fails or the process is terminated (e.g., with SIGSEGV for stack overflow).

Shared Memory Segment Growth

  • Memory Layout: The growth direction of shared memory segments depends on the system’s memory layout:
    • Legacy Layout: Uses bottom-up allocation, starting from TASK_UNMAPPED_BASE.
    • Non-Legacy Layout: Employs top-down allocation.
    • Determination: Functions like arch_pick_mmap_layout() and mmap_is_legacy() (on x86) decide this behavior.
  • Switching Layouts: You can switch to legacy mode using setarch -L.

Observing Segment Growth

  • Tool: The /proc/$$/maps file provides a snapshot of the process’s memory layout:
    • Bottom-Up vs. Top-Down:
      • If the dynamic linker (ld-linux) has a lower address than subsequent libraries, it indicates bottom-up allocation.
      • Otherwise, it’s top-down.
    • Practical Example: Comparing cat /proc/self/maps with setarch x86_64 -L cat /proc/self/maps can show the effect of switching between allocation strategies.