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.
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.
- Scope: This limit applies to all types of memory allocation within the process, including:
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
andsbrk
.
- 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.
- Components: This segment includes:
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()
andmmap_is_legacy()
(on x86) decide this behavior.
- Legacy Layout: Uses bottom-up allocation, starting from
- 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.
- If the dynamic linker (
- Practical Example: Comparing
cat /proc/self/maps
withsetarch x86_64 -L cat /proc/self/maps
can show the effect of switching between allocation strategies.
- Bottom-Up vs. Top-Down: