Re: [GSoC 2010][RESEND] Questions on Improved async/vector i/o support, fscache integration, and VFS change notification

On Sat, 3 Apr 2010 21:02:55 +0200
Mohammed Gamal <[email protected]> wrote:

> On Sat, Apr 3, 2010 at 12:46 PM, Jeff Layton <[email protected]> wrote:
> > On Sat, 3 Apr 2010 12:16:34 +0200
> > Mohammed Gamal <[email protected]> wrote:
> ... <snip> ...
> 
> >> - VFS change notification: My understanding on a very high level is
> >> that a mechanism is needed for the smb server to respond to
> >> inotfiy/dnotify calls, push these calls to the clients who have the
> >> corresponding shares mounted, and then reflect these changes on their
> >> ends. Is that understanding correct? And which operations need to be
> >> done in kernel space? Which other need to be done in userspace? And
> >> what are the boundaries between the two?
> >>
> >
> > The CIFS/SMB protocol already supports asynchronous notification when a
> > file changes on the server (maybe directory too? not sure). This
> > project is about hooking CIFS up to this. The problem currently is that
> > the VFS doesn't call down into the lower filesystem when a program
> > wants to be notified of changes.
> >
> > So that would have to be added at the VFS layer first, and then CIFS
> > would have to turn that into a call to set up notifications on the
> > server. You'd also have to have CIFS do a fanotify event when a file on
> > the server changes. This is all kernel-space stuff.
> >
> 
> Since inotify/dnotify calls are not delivered through the VFS then how
> do do notifications get delivered to userland? and which component
> delivers notifications to it if it's not the VFS or the lower
> filesystem?
> 

No, I think you misunderstand. In order for a program to receive
inotify/dnotify events, it has to tell the kernel what it wants to
monitor (via inotify_add_watch, for instance). The problem is that the
kernel doesn't pass that info down to CIFS. So you'd have to add an
interface into the VFS to do that.

Such a interface used to exist, but it was poorly implemented and Al Viro
ripped it out. Truthfully, this is less a CIFS/SMB project and much
more of a project to add this interface to the VFS. Once that's done,
the CIFS part will be relatively easy.

-- 
Jeff Layton <[email protected]>