How to Ensure HugeTLB Memory Persists During Live Kernel Update

Introduction

Live kernel updates (also known as kexec handover or live patching) allow administrators to apply security fixes or upgrade the kernel without rebooting the entire system. However, this process traditionally does not preserve hugetlbfs memory—the large-page allocations used by performance-critical applications like databases and virtual machines. Without preservation, those applications lose their huge page pools during the live update, causing service disruption. In 2026, Pratyush Yadav led a session at the Linux Storage, Filesystem, Memory Management, and BPF Summit to address this gap. This guide walks you through the steps to configure and use the new HugeTLB persistence feature successfully.

What You Need

• A Linux kernel version 5.20 or later (with hugetlbfs preservation patches applied)
• Hugetlbfs configured in the kernel (CONFIG_HUGETLBFS=y)
• Administrative (root) access to the system
• A live-update orchestrator tool supporting kexec handover (e.g., kexec-tools version 2.0.25+)
• Existing huge page pools allocated via /proc/sys/vm/nr_hugepages or hugetlbfs mount
• Backup of critical data (avoid data loss during testing)

Step-by-Step Guide

Step 1: Verify Kernel and Tool Support

Before enabling preservation, confirm that your kernel includes the HugeTLB live-update patches. Run:

uname -r

Check that the version is 5.20 or higher. Next, verify that your live-update orchestrator supports the --preserve-hugetlb flag (or equivalent). For example, with kexec:

kexec --version

If the orchestrator is missing the flag, update to a recent release or apply a backported patch.

Step 2: Reserve Huge Page Pools

hugetlbfs memory must be reserved at boot time or dynamically. To ensure pools survive a kexec handover, they need to be persistent—meaning the pages are locked and mapped into the old kernel’s memory map. Allocate huge pages using:

echo 256 > /proc/sys/vm/nr_hugepages

Or mount a hugetlbfs filesystem and pre-allocate pages:

mount -t hugetlbfs none /mnt/huge
for i in $(seq 1 100); do touch /mnt/huge/page_$i; done

Record the total number of preserved pages you intend to carry over.

Step 3: Enable Memory Preservation in the Live Update

When initiating the live update, pass the preserve-hugetlb flag to the orchestrator. For example, with kexec:

kexec -l /boot/vmlinuz-new --initrd=/boot/initrd.img --preserve-hugetlb

This flag tells the new kernel to adopt the old kernel’s huge page table entries. If your orchestrator uses a different syntax (e.g., --hugepages), consult its documentation.

Step 4: Perform the Live Kernel Switch

After the new kernel is loaded with preservation enabled, execute the handover:

kexec -e

This command replaces the running kernel without a full reboot. During the transition, the new kernel receives a memory map that includes the persisted huge pages. The process typically takes a few seconds.

Step 5: Verify HugeTLB Persistence

Once the new kernel is active, confirm that your huge page pools are intact:

cat /proc/meminfo | grep HugePages_Total

Compare the value with your earlier allocation. Also check that applications can still access the mounted hugetlbfs filesystem. If everything matches, the preservation succeeded.

Step 6: Handle Failures Gracefully

If the preservation fails (e.g., HugePages_Total drops to zero), the live update still completes, but huge pages are lost. Restore them by re-allocating: echo 256 > /proc/sys/vm/nr_hugepages and re-mounting. In such cases, consider enabling a fallback script in the initramfs that reallocates huge pages automatically.

Step 7: Test and Automate

Before relying on this feature in production, set up a staging environment and run several live-update cycles. Use monitoring tools to track huge page counts. Once stable, integrate the --preserve-hugetlb flag into your automated deployment pipeline (e.g., Ansible, SaltStack).

Tips and Best Practices

• Allocate early: Reserve huge pages at boot time using kernel boot parameters (hugepagesz=2M hugepages=256) to avoid fragmentation that can prevent preservation.
• Use persistent hugetlbfs mounts: Mount your hugetlbfs filesystem with the remount option after handover to ensure proper metadata transfer.
• Monitor logs: Check /var/log/kern.log or dmesg for messages like hugetlb: preserving pages or hugetlb: preservation failed.
• Test with small pools first: Start with a handful of huge pages (e.g., 4) to validate the feature without risking large memory loss.
• Keep backups: Even with preservation, a bug in the new kernel could corrupt huge page content. Always snapshot critical data before a live update.
• Stay updated: The HugeTLB preservation feature is relatively new. Follow kernel mailing lists for fixes and improvements.

By following these steps, you can ensure that your performance-sensitive applications retain their huge page memory during live kernel updates, minimizing downtime and preserving service quality.