Revoking filesystems [was Re: Sysfs attributes racing withunregistration]

[Alan Stern]

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.

Also, even devices that aren't hot-unpluggable can fail.  The end 
result should be pretty much the same.

> If it isn't too hard to get the event up from the block device to the
> filesystem when the block device is uncermoniously removed I might just
> make the time to have hotunplug trigger a filesystem wide revoke on a
> filesystem like ext4.
> 
> In addition to sysfs we need the same logic in proc, sysctl, and uio.
> So it makes sense to move towards a common library that can do all of
> the hard bits.

Ted mentioned the need for a new "device removed" superblock method.  
Then each filesystem can add its own implementation as people get
around to it.

Alan Stern

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