Diagnosing and Resolving Linux Kernel Page Faults in User-Space Applications
Symptoms
A Linux system experiencing frequent kernel page faults in user-space applications may exhibit behaviors such as process crashes, unresponsiveness, or kernel logs filled with messages like "kernel: page fault in user space"
. These faults often manifest as segmentation violations (SIGSEGV) or bus errors (SIGBUS), and may be accompanied by dmesg
output showing PGD
or PTE
corruption warnings.
Root Cause
Page faults in user space typically stem from invalid memory accesses, such as dereferencing a null pointer, accessing freed memory, or unauthorized memory regions. Underlying causes include:
- Memory management unit (MMU) misconfigurations due to kernel modules or drivers.
- Corrupted page tables caused by hardware failures (e.g., faulty RAM or CPU caches).
- Kernel bugs in the
do_page_fault()
handler orarch/x86/mm/fault.c
. - Applications with incorrect memory alignment or use of deprecated APIs.
Diagnosis Tools
Use the following tools to identify the root cause:
dmesg
: To capture kernel-level page fault logs.gdb
orcrash
: To analyze crash dumps or core files.perf
: For profiling memory access patterns and identifying high fault rates.vmstat
andtop
: To monitor memory and CPU usage anomalies.cat /proc/
: To inspect the memory mappings of the problematic process./maps
Example Code
Consider a C program that triggers a page fault by accessing an invalid pointer:
#include <stdio.h>
#include <stdlib.h>
int main() {
int *ptr = NULL;
*ptr = 42; // Invalid memory access
return 0;
}
Compiling and running this program will generate a SIGSEGV
, which the kernel logs as a page fault in user space. The dmesg
output may show:
[12345.678901] BUG: kernel page fault at virtual address 000000000000002a
[12345.678902] PGD 0x1234567800000000
[12345.678903] PTEs: 0x0
[12345.678904] Oops: 0000 [#1] SMP
Step-by-Step Solution
1. Analyze Kernel Logs: Use dmesg
to identify the address of the failed access and the process responsible. For example:
dmesg | grep -i "page fault"
2. Reproduce the Fault in a Controlled Environment: Run the application under gdb
to pinpoint the location of the fault:
gdb ./vulnerable_app
(gdb) run
(gdb) bt
3. Inspect Memory Maps: Check /proc/
for the process to validate its address space layout:
cat /proc/$(pidof vulnerable_app)/maps
4. Check for Hardware Issues: Run memtester
or mprime
to rule out faulty RAM. For CPU cache issues, use cpuid
or dmidecode
to verify hardware health.
5. Debug the Code: Use gdb
or valgrind
to trace invalid memory accesses. For example:
valgrind ./vulnerable_app
6. Patch the Application: Repair invalid pointer usage, ensure memory is properly allocated, and validate API usage. Example fix:
#include <stdio.h>
#include <stdlib.h>
int main() {
int *ptr = malloc(sizeof(int));
if (ptr) {
*ptr = 42;
free(ptr);
}
return 0;
}
7. Validate Kernel Configuration: For kernel-related issues, inspect /boot/config-$(uname -r)
for MMU or page table settings. Recompile the kernel if necessary. Use crash
to analyze kernel memory structures:
crash -c /boot/vmlinux-$(uname -r) /var/crash/$(uname -r)/vmcore
8. Monitor with perf
: Profile the application to detect high page fault rates:
perf stat -e page-faults ./vulnerable_app
9. Update Drivers and Kernel: Ensure all kernel modules and drivers are compatible with the current kernel version. Use modinfo
to check module metadata:
modinfo
10. Test System Stability: After making changes, reboot and monitor logs for reproducibility. Use systemtap
or ebpf
to trace page faults dynamically.