Prism: An efficient file mapping mechanism across multiple namespaces in mobile systems

Abstract

Existing mobile operating systems, such as Android, employ a unidirectional file mapping model to share files across different components of the systems. For example, a real file stored in F2FS of the Kernel layer may be mapped to the abstract file system in the Framework layer and the distributed file system in the Application layer. In this model, the namespaces of the file systems in different layers maintain varied metadata for the same real file, with modifications only propagating from top to bottom. Thus, the metadata of a real file in different namespaces may be inconsistent once the underlying file systems updates the real file without notifying the mapping namespaces, leading to a “mapping invalidation” issue. In this paper, we construct a new bidirectional model for mapping files in mobile systems. Specifically, we design a mechanism called Prism to enable bidirectional file mapping among multiple namespaces transparently. The core idea is to introduce a Shared-Inode (SInode) structure in the system’s intermediate layer to manage shared essential metadata. Prism provides two synchronization modes, i.e., immediate synchronization mode and delayed synchronization mode, to update the metadata changes from the lower namespaces to the upper namespaces in different scenarios. We implement Prism in Android 13 and evaluate it with real applications. Extensive experimental results show that Prism effectively resolves the “mapping invalidation” issue, particularly for image re-compression tasks. Experimental results demonstrate that our design reduces CPU and memory overhead by nearly 50% compared to the original system while having a trivial impact on overall performance.