Re: removable media revalidation - udev vs. devfs or static /dev

Greg KH
2004-01-08T01:18:18Z

On Wed, Jan 07, 2004 at 05:07:33PM -0800, Martin J. Bligh wrote:
> --Greg KH <greg@kroah.com> wrote (on Wednesday, January 07, 2004 16:41:24 -0800):
>
> > On Wed, Jan 07, 2004 at 04:32:26PM -0800, Martin J. Bligh wrote:
> >> >> NOTE! We do have an alternative: if we were to just make block device
> >> >> nodes support "readdir" and "lookup", you could just do
> >> >>
> >> >> open("/dev/sda/1" ...)
> >> >>
> >> >> and it magically works right. I've wanted to do this for a long time, but
> >> >> every time I suggest allowing it, people scream.
> >> >
> >> > Hm, that would be nice. I don't remember seeing it being proposed
> >> > before, what are the main complaints people have with this?
> >>
> >> Couldn't the partitions go under "/dev/sdaX/{1,2,3}" and solve the same
> >> problem without doing magic on the devices?
> >
> > No, that's not the point. As discussed on irc, I think you now
> > understand the issue (partitions not being present, media changed
> > without kernel knowing about it, etc.)
>
> I think it is the point ... you don't need to scan the device until
> someone does "readdir" / "lookup" as Linus says above. Just this way
> there's no duplexing of the block devices, just a magic directory node
> (which might avoid the screams Linus mentions).
> And it gets rid of the polling crap.

The problem is the following:
- user plugs in their usb flash reader with no media in it
- the main block device is create, no partitions
- user plugs a flash stick/whatever into the reader
- kernel gets no notification of this event
- userspace gets no notification of this event

How can userspace know to open the main block device now? Require that
we put a big "Rescan media now" button on the desktop? That's one way,
but users are used to having to not do that.

If a daemon does the scanning every so often, the media can be
automatically mounted, and an application can pop up saying that it
found some pictures on the new device, do you want to open up your image
application?

That help?

greg k-h