Hi, On 4/16/2023 3:55 PM, Amir Goldstein wrote: > On Tue, Feb 28, 2023 at 5:47 AM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote: >> 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. > Hi Hou, > > IIUC, there is not much value in making this a cross track session. > Seems like an FS track session that has not much to do with BPF > development. > > Am I understanding correctly or are there any cross subsystem > interactions that need to be discussed? Yes. Although the patchset for file-system iterator is still not ready, but I think the BPF mechanisms for file-system iterator is ready, so a cross track session maybe unnecessary. > > Perhaps we can join you as co-speaker for Miklos' traditional > "fsinfo" session? Thanks. I am glad to be a co-speaker for fsinfo session. > > Thanks, > Amir. > >> [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 >> >> _______________________________________________ >> Lsf-pc mailing list >> Lsf-pc@xxxxxxxxxxxxxxxxxxxxxxxxxx >> https://lists.linuxfoundation.org/mailman/listinfo/lsf-pc
