linux-kernel - Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

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Date:   Wed, 12 Aug 2020 14:33:13 +0100
From:   David Howells <dhowells@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     dhowells@...hat.com, Karel Zak <kzak@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        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 (was: [GIT PULL] Filesystem Information)

Miklos Szeredi <miklos@...redi.hu> wrote:

> You said yourself, that what's really needed is e.g. consistent
> snapshot of a complete mount tree topology.  And to get the complete
> topology FSINFO_ATTR_MOUNT_TOPOLOGY and FSINFO_ATTR_MOUNT_CHILDREN are
> needed for *each* individual mount.

That's not entirely true.

FSINFO_ATTR_MOUNT_ALL can be used instead of FSINFO_ATTR_MOUNT_CHILDREN if you
want to scan an entire subtree in one go.  It returns the same record type.

The result from ALL/CHILDREN includes sufficient information to build the
tree.  That only requires the parent ID.  All the rest of the information
TOPOLOGY exposes is to do with propagation.

Now, granted, I didn't include all of the topology info in the records
returned by ALL/CHILDREN because I don't expect it to change very often.  But
you can check the event counter supplied with each record to see if it might
have changed - and then call TOPOLOGY on the ones that changed.

If it simplifies life, I could add the propagation info into ALL/CHILDREN so
that you only need to call ALL to scan everything.  It requires larger
buffers, however.

> Adding a few generic binary interfaces is okay.   Adding many
> specialized binary interfaces is a PITA.

Text interfaces are also a PITA, especially when you may get multiple pieces
of information returned in one buffer and especially when you throw in
character escaping.  Of course, we can do it - and we do do it all over - but
that doesn't make it efficient.

David