/virtual-memory-paging-and-tlb

name: virtual-memory-paging-and-tlb
description: Virtual memory skill for paging, page tables, and TLB. Use when explaining page faults, multi-level page tables, TLB behavior, or virtual vs physical addressing. Activates on queries about virtual memory, page table, TLB, page fault, mmap paging, or x86-64 paging.

Virtual Memory, Paging, and TLB

Purpose

Explain virtual memory: paging, multi-level page tables, TLB role, page faults, and address translation — bridging OS kernels, embedded MPU, and performance analysis.

When to Use

• Understanding `mmap`, `brk`, and demand paging
• Debugging segfaults and guard pages
• Huge pages / TLB pressure tuning
• Contrasting Cortex-M MPU with full MMU systems

Workflow

1. Translation overview

Virtual address (VA)
├── TLB lookup → physical on hit
└── TLB miss → page table walk → fill TLB
        ├── valid PTE → physical address
        └── invalid → page fault (OS handles)

2. Page table (x86-64 4-level example)

CR3 → PML4 → PDPT → PD → PT → physical frame

Linux on x86_64 uses 4 KiB pages (default) and optional 2 MiB / 1 GiB huge pages.

3. Page fault types (simplified)

| Fault | Typical cause | |-------|----------------| | Major | Disk read — file-backed page not in RAM | | Minor | Zero-fill or COW break | | Protection | User access to kernel page, W^X violation |

# Linux page fault stats
grep pgfault /proc/vmstat

4. TLB pressure

Large sparse address spaces + random pointer chasing → TLB misses dominate.

Mitigations:

• `mmap` huge pages (`MAP_HUGETLB`, `madvise(MADV_HUGEPAGE)`)
• Smaller working set / better locality
• `numactl --membind` for NUMA

5. Embedded contrast (Cortex-M)

Many MCUs use **MPU** (region-based) not full paging — no TLB, fixed region count. Application processors use MMU + OS.

See `skills/platform/riscv-privileged` for Sv39/Sv48.

6. Userspace inspection (Linux)

cat /proc/self/maps
pmap -x $$

7. Agent usage

/virtual-memory-paging-and-tlb Explain why 4 KiB random access hurts TLB and hugepage helps

Common Problems

| Symptom | Cause | Fix | |---------|-------|-----| | Segfault | Unmapped VA | Fix pointer; check `maps` | | Slow mmap workload | TLB thrashing | Huge pages; reduce regions | | COW spike after fork | Shared pages split on write | Expected; consider `MAP_POPULATE` | | W^X fault | JIT without mprotect dance | Separate RW and RX mappings | | Wrong phys on MCU | No MMU — linear map | Use linker script addresses |

Related Skills

• `skills/computer-architecture/memory-hierarchy-and-caches` — cache after translation
• `skills/kernel-dev/kernel-memory-management` — kernel page allocator
• `skills/kernel/os-dev-scratch` — build paging from scratch
• `skills/platform/riscv-privileged` — Sv39 page tables
• `skills/allocators/numa-programming` — NUMA and migration