file metadata via fs API

[Miklos Szeredi]

On Tue, Aug 18, 2020 at 10:53 PM Linus Torvalds
<torvalds at linux-foundation.org> wrote:

> Basically, I think a rough rule of thumb can and should be:
>
>  - stuff that the VFS knows about natively and fully is clearly pretty
> mount-agnostic and generic, and can be represented in whatever
> extended "struct statfs_x" directly.
>
>  - anything that is variable-format and per-fs should be expressed in
> the ASCII buffer
>
> Look at our fancy new fs_context - that's pretty much what it does
> even inside the kernel. Sure, we have "binary" fields there for core
> basic information ("struct dentry *root", but also things like flags
> with MNT_NOSUID), but the configuration stuff is ASCII that the
> filesystem can parse itself.
>
> Exactly because some things are very much specific to some
> filesystems, not generic things.
>
> So we fundamentally already have a mix of "standard FS data" and
> "filesystem-specific options", and it's already basically split that
> way: binary flag fields for the generic stuff, and ASCII text for the
> odd options.

Okay.

Something else:  do we want a separate statmount(2) or is it okay to
mix per-mount and per-sb attributes in the same syscall?

/proc/mounts concatenates mount and sb options (since it copies the
/etc/mtab format)

/proc/self/mountinfo separates per-mount and per-sb data into
different fields at least, but the fields themselves are mixed

If we are introducing completely new interfaces, I think it would make
sense to separate per-mount and per-sb attributes somehow.  Atomicity
arguments don't apply since they have separate locking.  And we
already have separate interfaces for configuring them...

Thanks,
Miklos