[LFS/MM TOPIC] Enabling file and directory change notification for network and cluster file systems

[Amir Goldstein]

On Tue, Jan 21, 2020 at 10:30 AM ronnie sahlberg
<ronniesahlberg at gmail.com> wrote:
>
> On Tue, Jan 21, 2020 at 5:48 PM Amir Goldstein <amir73il at gmail.com> wrote:
> >
> > On Tue, Jan 21, 2020 at 5:55 AM Steve French <smfrench at gmail.com> wrote:
> > >
> > > Currently the inotify interface in the kernel can only be used for
> > > local file systems (unlike the previous change notify API used years
> > > ago, and the change notify interface in Windows and other OS which is
> > > primarily of interest for network file systems).
> > >
> > > I wanted to discuss the VFS changes needed to allow inotify requests
> > > to be passed into file systems so network and cluster file systems (as
> > > an example in the SMB3 case this simply means sending a
> > > SMB3_CHANGE_NOTIFY request to the server, whether Samba or Cloud
> > > (Azure) or Mac or Windows or Network Appliance - all support the API
> > > on the server side, the problem is that the network or cluster fs
> > > client isn't told about the request to wait on the inotify event).
> > > Although user space tools can use file system specific ioctls to wait
> > > on events, it is obviously preferable to allow network and cluster
> > > file systems to wait on events using the calls which current Linux
> > > GUIs use.
> > >
> > > This would allow gnome file manager GUI for example to be
> > > automatically updated when a file is added to an open directory window
> > > from another remote client.
> > >
> > > It would also fix the embarrassing problem noted in the inotify man page:
> > >
> > > "Inotify  reports  only events that a user-space program triggers
> > > through the filesystem
> > >        API.  As a result, it does not catch remote events that occur
> > > on  network  filesystems."
> > >
> > > but that is precisely the types of notifications that are most useful
> > > ... users often are aware of updates to local directories from the
> > > same system, but ... automatic notifications that allow GUIs to be
> > > updated on changes from **other** clients is of more value (and this
> > > is exactly what the equivalent API allows on other OS).
> > >
> > > The changes to the Linux VFS are small.
> > >
> > >
> >
> > Miklos has already posted an RFC patch:
> > https://lore.kernel.org/linux-fsdevel/20190507085707.GD30899@veci.piliscsaba.redhat.com/
> >
> > Did you try it?
> >
> > You also did not answer Miklos' question:
> > does the smb protocol support whole filesystem (or subtree) notifications?
> > (or just per-directory notifications)?
>
> SMB can do both. There is a flag that specifies if you want to just
> get notified about the directory itself
> or whether  you want notifications from the whole subtree.
>

I see. There is no user API in Linux to request a "subtree" watch.
For the private case that the user requests a FAN_MARK_FILESYSTEM,
cifs may translate that into a SMB2_WATCH_TREE for the share root dir.

For that, Miklos' RFC of vfs interface inode->i_op->notify_update(inode)
should be enriched with sb->s_op->notify_update(sb).

Thanks,
Amir.