linux-kernel - Re: file metadata via fs API

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Message-ID: <CAJfpegsBSsMkSXReN6Sheye1cksCO2pcqcx_3VwY4C1J9kDhaw@mail.gmail.com>
Date:   Fri, 21 Aug 2020 15:17:59 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steven Whitehouse <swhiteho@...hat.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>, Karel Zak <kzak@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API

On Tue, Aug 18, 2020 at 10:53 PM Linus Torvalds
<torvalds@...ux-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