DAXFS: A Lock-Free Shared Filesystem for CXL Disaggregated Memory

TL;DR

DaxFS is a lock-free Linux filesystem designed for CXL shared memory, enabling multi-host coordination and high throughput without centralized control, validated through emulation and microbenchmarks.

Contribution

It introduces a lock-free, CAS-based filesystem for CXL shared memory that supports multi-host concurrency and demand-paged caching with decentralized victim selection.

Findings

Maintains >99% CAS accuracy under contention with no lost updates.

Achieves up to 2.68x higher random write throughput than tmpfs.

Extends lock-free design to GPU page cache operations at PCIe 5.0 bandwidth.

Abstract

CXL (Compute Express Link) enables multiple hosts to share byte-addressable memory with hardware cache coherence, but no existing filesystem exploits this for lock-free multi-host coordination. We present DaxFS, a Linux filesystem for CXL shared memory that uses cmpxchg atomic operations, which CXL makes coherent across host boundaries, as its sole coordination primitive. A CAS-based hash overlay enables lock-free concurrent writes from multiple hosts without any centralized coordinator. A cooperative shared page cache with a novel multi-host clock eviction algorithm (MH-clock) provides demand-paged caching in shared DAX memory, with fully decentralized victim selection via cmpxchg. We validate multi-host correctness using QEMU-emulated CXL 3.0, where two virtual hosts share a memory region with TCP-forwarded atomics. Under cross-host contention, DaxFS maintains >99% CAS accuracy with no lost updates. On single-host DRAM-backed DAX, DaxFS exceeds tmpfs throughput across all write workloads, achieving up to 2.68x higher random write throughput with 4 threads and 1.18x higher random read throughput at 64 KB. Preliminary GPU microbenchmarks show that the cmpxchg-based design extends to GPU threads performing page cache operations at PCIe 5.0 bandwidth limits.