linux-kernel - Re: [RFC PATCH 0/3] quering mount attributes

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Message-ID: <CAOQ4uxgHxVqtvb51Z27Sgft-U=oYtXeiv+3HJbara4zdRC-FZg@mail.gmail.com>
Date:   Fri, 15 Sep 2023 06:06:31 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Ian Kent <raven@...maw.net>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>
Subject: Re: [RFC PATCH 0/3] quering mount attributes

On Fri, Sep 15, 2023 at 4:20 AM Ian Kent <raven@...maw.net> wrote:
>
> On 14/9/23 14:47, Amir Goldstein wrote:
> > On Wed, Sep 13, 2023 at 6:22 PM Miklos Szeredi <mszeredi@...hat.com> wrote:
> >> Implement the mount querying syscalls agreed on at LSF/MM 2023.  This is an
> >> RFC with just x86_64 syscalls.
> >>
> >> Excepting notification this should allow full replacement for
> >> parsing /proc/self/mountinfo.
> > Since you mentioned notifications, I will add that the plan discussed
> > in LFSMM was, once we have an API to query mount stats and children,
> > implement fanotify events for:
> > mount [mntuid] was un/mounted at [parent mntuid],[dirfid+name]
> >
> > As with other fanotify events, the self mntuid and dirfid+name
> > information can be omitted and without it, multiple un/mount events
> > from the same parent mntuid will be merged, allowing userspace
> > to listmnt() periodically only mntuid whose child mounts have changed,
> > with little risk of event queue overflow.
> >
> > The possible monitoring scopes would be the entire mount namespace
> > of the monitoring program or watching a single mount for change in
> > its children mounts. The latter is similar to inotify directory children watch,
> > where the watches needs to be set recursively, with all the weight on
> > userspace to avoid races.
>
> It's been my belief that the existing notification mechanisms don't
> quite fully satisfy the needs of users of these calls (aka. the need
> I found when implementing David's original calls into systemd).
>
> Specifically the ability to process a batch of notifications at once.
>
> Admittedly the notifications mechanism that David originally implemented
> didn't fully implement what I found I needed but it did provide for a
> settable queue length and getting a batch of notifications at a time.
>
> Am I mistaken in my belief?
>

I am not sure I understand the question.

fanotify has an event queue (16K events by default), but it can
also use unlimited size.
With a limited size queue, event queue overflow generates an
overflow event.

event listeners can read a batch of events, depending on
the size of the buffer that they provide.

when multiple events with same information are queued,
for example "something was un/mounted over parent mntuid 100"
fanotify will merged those all those events in the queue and the
event listeners will get only one such event in the batch.

> Don't misunderstand me, it would be great for the existing notification
> mechanisms to support these system calls, I just have a specific use case
> in mind that I think is important, at least to me.
>

Please explain the use case and your belief about existing fanotify
limitations. I did not understand it.

Thanks,
Amir.