linux-kernel - Re: [RFC PATCH 0/7] Inotify support in FUSE and virtiofs

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Date:   Tue, 2 Nov 2021 12:09:31 +0100
From:   Jan Kara <jack@...e.cz>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        Ioannis Angelakopoulos <iangelak@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        virtio-fs-list <virtio-fs@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Steve French <sfrench@...ba.org>
Subject: Re: [RFC PATCH 0/7] Inotify support in FUSE and virtiofs

On Wed 27-10-21 16:29:40, Vivek Goyal wrote:
> On Wed, Oct 27, 2021 at 03:23:19PM +0200, Jan Kara wrote:
> > On Wed 27-10-21 08:59:15, Amir Goldstein wrote:
> > > On Tue, Oct 26, 2021 at 10:14 PM Ioannis Angelakopoulos
> > > <iangelak@...hat.com> wrote:
> > > > On Tue, Oct 26, 2021 at 2:27 PM Vivek Goyal <vgoyal@...hat.com> wrote:
> > > > The problem here is that the OPEN event might still be travelling towards the guest in the
> > > > virtqueues and arrives after the guest has already deleted its local inode.
> > > > While the remote event (OPEN) received by the guest is valid, its fsnotify
> > > > subsystem will drop it since the local inode is not there.
> > > >
> > > 
> > > I have a feeling that we are mixing issues related to shared server
> > > and remote fsnotify.
> > 
> > I don't think Ioannis was speaking about shared server case here. I think
> > he says that in a simple FUSE remote notification setup we can loose OPEN
> > events (or basically any other) if the inode for which the event happens
> > gets deleted sufficiently early after the event being generated. That seems
> > indeed somewhat unexpected and could be confusing if it happens e.g. for
> > some directory operations.
> 
> Hi Jan,
> 
> Agreed. That's what Ioannis is trying to say. That some of the remote events
> can be lost if fuse/guest local inode is unlinked. I think problem exists
> both for shared and non-shared directory case.
> 
> With local filesystems we have a control that we can first queue up
> the event in buffer before we remove local watches. With events travelling
> from a remote server, there is no such control/synchronization. It can
> very well happen that events got delayed in the communication path
> somewhere and local watches went away and now there is no way to 
> deliver those events to the application.

So after thinking for some time about this I have the following question
about the architecture of this solution: Why do you actually have local
fsnotify watches at all? They seem to cause quite some trouble... I mean
cannot we have fsnotify marks only on FUSE server and generate all events
there? When e.g. file is created from the client, client tells the server
about creation, the server performs the creation which generates the
fsnotify event, that is received by the server and forwared back to the
client which just queues it into notification group's queue for userspace
to read it.

Now with this architecture there's no problem with duplicate events for
local & server notification marks, similarly there's no problem with lost
events after inode deletion because events received by the client are
directly queued into notification queue without any checking whether inode
is still alive etc. Would this work or am I missing something?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR