Sandbox cgroup silently disables transparent huge pages (THP)

Problem

Processes launched by Claude Code cannot use transparent huge pages (THP), even when the host system has transparent_hugepage/enabled=always and defrag=always. The same binary run from a regular user shell gets full THP promotion; run from Claude Code it gets zero.

This silently breaks performance measurement and verification of THP-related optimizations.

Reproduction

1. Set up THP on the host:

sudo sh -c 'echo always > /sys/kernel/mm/transparent_hugepage/enabled'
sudo sh -c 'echo always > /sys/kernel/mm/transparent_hugepage/defrag'

1. Compile and run this test program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#define SIZE (10 * 1024 * 1024)
int main() {
    void *p = mmap(NULL, SIZE, PROT_READ|PROT_WRITE,
                   MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
    if (p == MAP_FAILED) { perror("mmap"); return 1; }
    madvise(p, SIZE, MADV_HUGEPAGE);
    memset(p, 0, SIZE);
    char cmd[128];
    snprintf(cmd, sizeof(cmd),
        "grep AnonHugePages /proc/%d/smaps | awk '{s+=$2}END{printf \"AnonHugePages: %%d kB\\n\",s}'",
        getpid());
    system(cmd);
    munmap(p, SIZE);
    return 0;
}

1. Run from a regular shell → AnonHugePages: 10240 kB (correct)
2. Run from Claude Code → AnonHugePages: 0 kB (broken)

madvise(MADV_HUGEPAGE) returns 0 (success) in both cases, so the caller has no way to detect the failure.

Likely cause

The Claude Code sandbox likely runs processes in a cgroup (v2) that has THP disabled. This could be:

• memory.numa_stat or a cgroup-level THP restriction
• A seccomp filter that allows madvise but neuters MADV_HUGEPAGE
• A memory cgroup controller setting that prevents hugepage allocation

Impact

• Cannot verify THP-related performance optimizations from within Claude Code
• Benchmarks run from Claude Code show different memory behavior than the user's environment
• madvise returning success when the hint is silently ignored makes debugging very difficult

Environment

• Host: Ubuntu, kernel 7.0.0-12-generic
• CPU: AMD Ryzen AI Max+ 395
• Claude Code: CLI (latest)

Suggestion

If THP must be disabled in the sandbox for resource management reasons, it would help if madvise(MADV_HUGEPAGE) returned an error instead of silently succeeding. Alternatively, documenting this limitation would save users from debugging phantom performance differences.