Date:   Tue, 04 Aug 2020 10:15:47 +0800
From:   Ian Kent <raven@...maw.net>
To:     Miklos Szeredi <miklos@...redi.hu>,
        David Howells <dhowells@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.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>,
        linux-fsdevel@...r.kernel.org,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] Filesystem Information

On Mon, 2020-08-03 at 18:42 +0200, Miklos Szeredi wrote:
> On Mon, Aug 3, 2020 at 5:50 PM David Howells <dhowells@...hat.com>
> wrote:
> > 
> > Hi Linus,
> > 
> > Here's a set of patches that adds a system call, fsinfo(), that
> > allows
> > information about the VFS, mount topology, superblock and files to
> > be
> > retrieved.
> > 
> > The patchset is based on top of the mount notifications patchset so
> > that
> > the mount notification mechanism can be hooked to provide event
> > counters
> > that can be retrieved with fsinfo(), thereby making it a lot faster
> > to work
> > out which mounts have changed.
> > 
> > Note that there was a last minute change requested by Miklós: the
> > event
> > counter bits got moved from the mount notification patchset to this
> > one.
> > The counters got made atomic_long_t inside the kernel and __u64 in
> > the
> > UAPI.  The aggregate changes can be assessed by comparing pre-
> > change tag,
> > fsinfo-core-20200724 to the requested pull tag.
> > 
> > Karel Zak has created preliminary patches that add support to
> > libmount[*]
> > and Ian Kent has started working on making systemd use these and
> > mount
> > notifications[**].
> 
> So why are you asking to pull at this stage?
> 
> Has anyone done a review of the patchset?

I have been working with the patch set as it has evolved for quite a
while now.

I've been reading the kernel code quite a bit and forwarded questions
and minor changes to David as they arose.

As for a review, not specifically, but while the series implements a
rather large change it's surprisingly straight forward to read.

In the time I have been working with it I haven't noticed any problems
except for those few minor things that I reported to David early on (in
some cases accompanied by simple patches).

And more recently (obviously) I've been working with the mount
notifications changes and, from a readability POV, I find it's the
same as the fsinfo() code.

> 
> I think it's obvious that this API needs more work.  The integration
> work done by Ian is a good direction, but it's not quite the full
> validation and review that a complex new API needs.

Maybe but the system call is fundamental to making notifications useful
and, as I say, after working with it for quite a while I don't fell
there's missing features (that David hasn't added along the way) and
have found it provides what's needed for what I'm doing (for mount
notifications at least).

I'll be posting a github PR for systemd for discussion soon while I
get on with completing the systemd change. Like overflow handling and
meson build system changes to allow building with and without the
util-linux libmount changes.

So, ideally, I'd like to see the series merged, we've been working on
it for quite a considerable time now.

Ian

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