Re: [Lsf-pc] [LSF/MM/BPF TOPIC] bpf iterator for file-system

[Amir Goldstein <amir73il@gmail.com>]

On Tue, Feb 28, 2023 at 5:47 AM Hou Tao <houtao@huaweicloud.com> 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?

Perhaps we can join you as co-speaker for Miklos' traditional
"fsinfo" session?

Thanks,
Amir.

>
> [0]:
> https://lore.kernel.org/linux-fsdevel/YnEeuw6fd1A8usjj@miu.piliscsaba.redhat.com/
> [1]: https://lore.kernel.org/linux-mm/20230219073318.366189-1-nphamcs@gmail.com/
> [2]:
> https://lore.kernel.org/linux-fsdevel/CAJfpegsCKEx41KA1S2QJ9gX9BEBG4_d8igA0DT66GFH2ZanspA@mail.gmail.com/
> [3]: https://lore.kernel.org/bpf/20200509175859.2474608-1-yhs@fb.com/
> [4]: https://docs.kernel.org/bpf/bpf_iterators.html
>
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