Keyboard shortcuts

Press ← or → to navigate between chapters

Press S or / to search in the book

Press ? to show this help

Press Esc to hide this help

Memory Management Syscalls

This document describes the memory management system calls (syscalls) in the Kyronix kernel. It is the child of Syscalls.

Syscall Table

  • 9 mmap - Map memory or files into address space

  • 10 mprotect - Set memory protection on a region

  • 11 munmap - Unmap memory from address space

  • 12 brk - Change data segment size

  • 25 mremap - Remap a virtual memory area (VMA)

  • 26 msync - Synchronize memory with storage (noop for ramfs)

  • 27 mincore - Determine page residency in memory

  • 28 madvise - Provide advice about memory usage (noop)

  • 29 shmget - Get System V shared memory segment

  • 30 shmat - Attach shared memory to address space

  • 31 shmctl - Control shared memory operations

  • 67 shmdt - Detach shared memory from address space

mmap Flags

The mmap syscall accepts the following flags:

  • MAP_ANON - Create an anonymous mapping (not backed by a file)
  • MAP_FIXED - Place the mapping at the exact address specified
  • MAP_PRIVATE - Create a private copy-on-write (COW) mapping
  • MAP_SHARED - Create a shared mapping visible to other processes

mmap Details

The mmap syscall uses mmap_pick_addr() to select a virtual address for anonymous mappings. The address selection uses a bump allocator that starts at:

p->mmap_bump = 0x0000500000000000 + random_offset

The mmap implementation supports:

  • Anonymous mappings (file descriptor is -1)
  • File-backed mappings (valid file descriptor)
  • Character device custom mappings (User I/O (UIO))

brk Details

The brk syscall implements the classic break mechanism for heap management:

  • Operates with page-granularity (minimum allocation is one page)
  • Allocates zeroed pages via pmm_alloc_zeroed()
  • Tracks pages_alloc and pages_freed counters
  • Expands the data segment by mapping new pages
  • Contracts the data segment by unmapping pages

mprotect Details

The mprotect syscall changes both:

  • Virtual Memory Area (VMA) metadata (protection flags)
  • Page table entries (hardware-level permissions)

Shared Memory (SHM)

The SHM syscalls implement System V shared memory with the following characteristics:

  • Requires JAILF_IPC flag for jail isolation
  • Maximum of 64 shared memory segments per process
  • Maximum of 4096 pages (16 MiB at 4 KiB pages) per segment
  • Supports attach, detach, and control operations

Last reviewed: 2026-07-22