Kernel/User-level Collaborative Persistent Memory File System with Efficiency and Protection

Emerging high performance non-volatile memories recall the importance of efficient file system design. To avoid the virtual file system (VFS) and syscall overhead as in these kernel-based file systems, recent works deploy file systems directly in user level. Unfortunately, a userlevel file system can easily be corrupted by a buggy program with misused pointers, and is hard to scale on multi-core platforms which incorporates a centralized coordination service. In this paper, we propose KucoFS, a Kernel and user-level collaborative file system. It consists of two parts: a user-level library with direct-access interfaces, and a kernel thread, which performs metadata updates and enforces write protection by toggling the permission bits in the page table. Hence, KucoFS achieves both direct-access of user-level designs and fine-grained write protection of kernel-level ones. We further explore its scalability to multicores: For metadata scalability, KucoFS rebalances the pathname resolution overhead between the kernel and userspace, by adopting the index offloading technique. For data access efficiency, it coordinates the data allocation between kernel and userspace, and uses range-lock write and lock-free read to improve concurrency. Experiments on Optane DC persistent memory show that KucoFS significantly outperforms existing file systems and shows better scalability.