Linux 7.1 to Retire UDP-Lite – Allows for Better Performance with Cleansed Code

Source: Hacker News

Background

The upcoming Linux 7.1 kernel cycle is set to retire UDP‑Lite support. UDP‑Lite allowed partial checksums so that potentially damaged or corrupted packets could still be delivered to the application. Since the Linux 2.6.20 days there has been UDP‑Lite support, but the kernel is now set to retire it due to long‑standing breakage and the opportunity to improve performance for non‑UDP‑Lite users.

Deprecation Details

Kuniyuki Iwashima of Amazon/AWS explained the issues that went unchecked for years and the performance benefit of removing UDP‑Lite from the kernel:

In 2023, syzbot found a null‑ptr‑deref bug triggered when UDP‑Lite attempted to charge an skb after the total memory usage for UDP‑Lite and UDP exceeded a system‑wide threshold, net.ipv4.udp_mem.
Since this threshold is shared with UDP, the bug would have been easy to trigger if any real‑world applications were using UDP‑Lite; however, only syzbot ever stumbled upon it.
The bug had persisted since 2016, suggesting that UDP‑Lite had remained unused for 7 years at that point.
The bug was fixed in commit ad42a35bdfc6 (“udplite: Fix NULL pointer dereference in __sk_mem_raise_allocated().”), and another commit be28c14ac8bb (“udplite: Print deprecation notice.”) announced the deprecation plan.
Since then, no one has complained, so it is time to officially retire UDP‑Lite.

The deprecation process first removes IPv6 and IPv4 UDP‑Lite sockets, then gradually cleans up the remaining dead/unnecessary code within the UDP stack.

Performance Impact

By removing a bunch of conditionals for UDP‑Lite from the fast path, udp_rr with 20 000 flows sees a 10 % increase in packets per second (13.3 Mpps → 14.7 Mpps) on an AMD EPYC 7B12 (Zen 2) 64‑Core Processor platform.

With FDO, the baseline is much higher and the delta was ~3 % (20.1 Mpps → 20.7 Mpps).

Thus, the UDP‑Lite code hasn’t been used in years, and its removal can net a measurable (+3 % – 10 %) improvement for other UDP workloads.

Timeline

• 2023 – Deprecation notice added, calling for removal in 2025.
• 2025 – Linux 6.18 LTS released; no complaints received.
• 2026 – Removal proceeds as part of the Linux 7.1 cycle.

Merge Reference

This merge commit in net-next retires UDP‑Lite with the plan to merge it for Linux 7.1.