Add FreePages to HugePagesInfo for hugepage availability reporting

[srikalyan]

Summary

• Adds FreePages *uint64 field to HugePagesInfo struct, populated from /sys/devices/system/node/node<N>/hugepages/hugepages-<size>kB/free_hugepages
• Uses pointer type with omitempty to distinguish between "0 free pages" and "data unavailable"
• Enables consumers like the Kubernetes Memory Manager to verify actual hugepage availability during pod admission

Motivation

The Kubernetes Static Memory Manager currently only tracks hugepage allocations for Guaranteed QoS pods. However, Burstable and BestEffort pods can consume hugepages (via hugetlbfs mounts or mmap with MAP_HUGETLB) without being tracked. This causes Guaranteed pods to be admitted based on stale allocation data, only to fail at runtime when hugepages are exhausted.

By exposing free_hugepages from sysfs, consumers can verify actual OS-reported availability before making admission decisions.

Design

The field uses *uint64 with omitempty (following v2 convention) to distinguish:

• nil: free_hugepages data unavailable (file missing or unreadable)
• 0: zero free hugepages available
• N: N free hugepages available

This allows consumers to detect when the data isn't available and fall back appropriately.

Note: Since GetMachineInfo() is cached at startup, the FreePages value represents point-in-time data. Consumers requiring real-time availability may need to read sysfs directly or use a dedicated fresh-read method (pending KEP outcome).

Test Plan

• Added unit tests for GetHugePagesFree() in sysfs
• Updated TestGetHugePagesInfo to verify FreePages is correctly populated
• Verified JSON serialization with omitempty behavior
• All existing tests pass

Related

• KEP: Memory Manager Hugepages Availability Verification kubernetes/enhancements#5759 (KEP-5759: Memory Manager Hugepages Availability Verification)
• Kubernetes Issue: Static Memory Manager doesn't verify OS reported available huge pages during pod admission kubernetes/kubernetes#134395
• KEP PR: KEP-5759: Memory Manager Hugepages Availability Verification kubernetes/enhancements#5753

[srikalyan]

Based on KEP review feedback, I'm considering changing FreePages from *uint64 to uint64.

Rationale: On Linux systems with hugepages configured, the sysfs interface (/sys/devices/system/node/node<N>/hugepages/hugepages-<size>kB/free_hugepages) is always available. We don't need to distinguish between "0 free hugepages" and "data unavailable" since sysfs won't be unavailable.

Current implementation: Uses *uint64 with omitempty to distinguish nil (unavailable) from 0 (zero free).

Proposed change: Use plain uint64. A value of 0 simply means zero free hugepages.

What are your thoughts on this? I'm happy to update the PR either way based on cadvisor's conventions and your preference.

cc @iwankgb