>From time to time, new syscalls have been proposed to gain more observability for file-system: (1) getvalues() [0]. It uses a hierarchical namespace API to gather and return multiple values in single syscall. (2) cachestat() [1]. It returns the cache status (e.g., number of dirty pages) of a given file in a scalable way. All these proposals requires adding a new syscall. Here I would like to propose another solution for file system observability: bpf iterator for file system object. The initial idea came when I was trying to implement a filefrag-like page cache tool with support for multi-order folio, so that we can know the number of multi-order folios and the orders of those folios in page cache. After developing a demo for it, I realized that we could use it to provide more observability for file system objects. e.g., dumping the per-cpu iostat for a super block [2], iterating all inodes in a super-block to dump info for specific inodes (e.g., unlinked but pinned inode), or displaying the flags of a specific mount. The BPF iterator was introduced in v5.8 [3] to support flexible content dumping for kernel objects. It works by creating bpf iterator file [4], which is a seq-like read-only file, and the content of the bpf iterator file is determined by a previously loaded bpf program, so userspace can read the bpf iterator file to get the information it needs. However there are some unresolved issues: (1) The privilege. Loading the bpf program requires CAP_ADMIN or CAP_BPF. This means that the observability will be available to the privileged process. Maybe we can load the bpf program through a privileged process and make the bpf iterator file being readable for normal users. (2) Prevent pinning the super-block In the current naive implementation, the bpf iterator simply pins the super-block of the passed fd and prevents the super-block from being destroyed. Perhaps fs-pin is a better choice, so the bpf iterator can be deactivated after the filesystem is umounted. I hope to send out an RFC soon before LSF/MM/BPF for further discussion. [0]: https://lore.kernel.org/linux-fsdevel/YnEeuw6fd1A8usjj@xxxxxxxxxxxxxxxxxxxxxxxxx/ [1]: https://lore.kernel.org/linux-mm/20230219073318.366189-1-nphamcs@xxxxxxxxx/ [2]: https://lore.kernel.org/linux-fsdevel/CAJfpegsCKEx41KA1S2QJ9gX9BEBG4_d8igA0DT66GFH2ZanspA@xxxxxxxxxxxxxx/ [3]: https://lore.kernel.org/bpf/20200509175859.2474608-1-yhs@xxxxxx/ [4]: https://docs.kernel.org/bpf/bpf_iterators.html
