How to use
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npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill virtual-memory-paging-and-tlbBefore you start
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Original skill document
Rendered from the source repository. Headings, examples, code, tables, links, and referenced images are preserved.
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 frameLinux 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 |
bash
# Linux page fault stats
grep pgfault /proc/vmstat4. TLB pressure
Large sparse address spaces + random pointer chasing → TLB misses dominate.
Mitigations:
mmaphuge pages (MAP_HUGETLB,madvise(MADV_HUGEPAGE))- Smaller working set / better locality
numactl --membindfor 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)
bash
cat /proc/self/maps
pmap -x $$7. Agent usage
/virtual-memory-paging-and-tlb Explain why 4 KiB random access hurts TLB and hugepage helpsCommon 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 translationskills/kernel-dev/kernel-memory-management— kernel page allocatorskills/kernel/os-dev-scratch— build paging from scratchskills/platform/riscv-privileged— Sv39 page tablesskills/allocators/numa-programming— NUMA and migration
