linux-kernel - Re: Revoking filesystems [was Re: Sysfs attributes racing with unregistration]

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Date:	Thu, 5 Jan 2012 10:18:57 -0800
From:	Greg KH <gregkh@...e.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Theodore Ts'o <tytso@....edu>, Tejun Heo <tj@...nel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: Revoking filesystems [was Re: Sysfs attributes racing with
 unregistration]

On Thu, Jan 05, 2012 at 10:13:31AM -0500, Alan Stern wrote:
> On Wed, 4 Jan 2012, Eric W. Biederman wrote:
> 
> > > Ted Ts'o has been talking about something similar but not the same -- a
> > > way to revoke an entire filesystem.  For example, see commit
> > > 7c2e70879fc0949b4220ee61b7c4553f6976a94d (ext4: add ext4-specific
> > > kludge to avoid an oops after the disk disappears).
> > >
> > > The use case for that is obvious and widespread: Somebody yanks out a
> > > USB drive without unmounting it first.
> > 
> > Agreed.  The best I have at the moment is a library that can wrap
> > filesystem methods to implement the hotplug bits.
> > 
> > Do you know how hard it is to remove event up to the filesystem that
> > sits on top of a block device?
> 
> I don't have a clear idea of what's involved (in particular, how to go
> from a block_device structure to a mounted filesystem).  But the place
> to do it would probably be block/genhd.c:invalidate_partition().  Ted
> can tell you if there's a better alternative.
> 
> > Do you know how hard it is to detect at mount time if a block device
> > might be hot-plugable?  We can always use a mount option here and
> > make userspace figure it out, but being to have a good default would
> > be nice.
> 
> I don't think it's possible to tell if a device is hot-unpluggable.  
> For example, the device itself might not be removable from its parent, 
> but the parent might be hot-unpluggable.  You'll probably have to 
> assume that every device can potentially be unplugged, one way or 
> another.

These days, _any_ block device is hot unplugable, what with PCI hotplug
and the like (running in a virtual machine, etc.)  So you always need to
assume that any device can go away at any point in time.

thanks,

greg k-h
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