Exploring Operating Systems

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.

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.

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