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

From: David Howells <dhowells@redhat.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: dhowells@redhat.com, Karel Zak <kzak@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@redhat.com>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Nicolas Dichtel <nicolas.dichtel@6wind.com>,
	Christian Brauner <christian@brauner.io>,
	Lennart Poettering <lennart@poettering.net>,
	Linux API <linux-api@vger.kernel.org>,
	Ian Kent <raven@themaw.net>,
	LSM <linux-security-module@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)
Date: Wed, 12 Aug 2020 14:33:13 +0100	[thread overview]
Message-ID: <133508.1597239193@warthog.procyon.org.uk> (raw)
In-Reply-To: <CAJfpegs4gzvJMBz=su8KgXXxX41tv8tVhO88Eap9pDeHRaSDPA@mail.gmail.com>

Miklos Szeredi <miklos@szeredi.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