linux_kernel_portfolio

Bootmem Trace: Page Allocation and Refcount Bug

Files

File	Purpose
bootmem_trace.c	Kernel module source with axiomatic comments
worksheet.md	Step-by-step hand calculations and error report
Makefile	Build configuration

Build and Run

cd kernel_exercises/bootmem_trace
make
cat /proc/buddyinfo  # Record before
sudo insmod bootmem_trace.ko
dmesg | tail -30     # Observe values
sudo rmmod bootmem_trace
cat /proc/buddyinfo  # Compare after

Exercise Goal

1. Trace page allocation from axioms
2. Calculate PFN, physical address, zone by hand
3. Trace _refcount transitions: 1→2→1→0→-1
4. Observe kernel BUG on refcount underflow

Worksheet Contents

• Lines 01-30: RAM, byte, page, PFN definitions
• Lines 31-51: vmemmap, zone definitions
• Lines 52-72: _refcount, GFP_KERNEL derivation
• Lines 73-91: Code mapping and execution
• Lines 92-144: Error report and brain analysis