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

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jan 05, 2012 at 01:27:52PM -0500, Ted Ts'o wrote:
> Ideally, we should do both.  The block device should call a
> notification function (probably run out of a workqueue context, to
> avoid locking issues) which tells the file system, "the block device
> is _gone_ and isn't coming back".  Any attempts to read or write to
> the block device should return errors, since there maybe writeback
> happening in the background while the file system is shutting down
> file system mount.  Once the file system is done, it can all a
> function which tells the block device layer that it's OK to release
> the block device and its related structures.

FYI: we have all the functionality for that available in XFS and would
just need to wire it up.  It's also triggered if we get a write I/O
error for metadata (typically the log), so with a minim delay we
actually provide that behaviour already.

> In order for the file system to shut down the file system cleanly, it
> will need to access VFS-level revoke functionality that replaces file
> descriptors with ones that returns an error on reads and writes, and
> which does the right thing with mmap's[1], etc.

And that part is close to impossible to get right.