Use /proc/self/pagemap to Inspect Your Process Layout

Inspecting Your Own Mappings

/proc/self/pagemap exposes how the kernel maps each virtual page: PFN, present, swapped, soft-dirty bits. It is the userspace microscope for paging behavior without writing a kernel module.

Entry Format

Each 8-byte entry per 4 KB page:

• Bit 63: present
• Bits 0-54: PFN if present
• Other flags: swapped, file-backed, exclusive

For example, reading pagemap for your stack VA tells you the exact physical frame backing the current stack page after a fault.

Why This Matters in Production

Tools like smem, pmap, and Chrome's about://memory-internals all read /proc/PID/pagemap to report real per-process memory usage instead of guessing from virtual address space size. Getting a bit offset wrong , present is bit 62, not 61 , silently reports swapped-out pages as resident, exactly the kind of bug that makes memory-usage dashboards lie to operators.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will mmap regions, touch pages selectively, and decode pagemap entries to print a map of resident pages. The task asks you to confirm demand paging by seeing present bits flip only after access.