Re: [PATCH v4 1/6] fs: enhance and rename shutdown() callback to remove_bdev()

Jan Kara
2025-07-10T13:10:11Z

On Wed 09-07-25 13:49:12, Kent Overstreet wrote:
> On Wed, Jul 09, 2025 at 07:23:07PM +0200, Jan Kara wrote:
> > > It also avoids the problem of ->mark_dead events being generated
> > > from a context that holds filesystem/vfs locks and then deadlocking
> > > waiting for those locks to be released.
> > >
> > > IOWs, a multi-device filesystem should really be implementing
> > > ->mark_dead itself, and should not be depending on being able to
> > > lock the superblock to take an active reference to it.
> > >
> > > It should be pretty clear that these are not issues that the generic
> > > filesystem ->mark_dead implementation should be trying to
> > > handle.....
> >
> > Well, IMO every fs implementation needs to do the bdev -> sb transition and
> > make sb somehow stable. It may be that grabbing s_umount and active sb
> > reference is not what everybody wants but AFAIU btrfs as the second
> > multi-device filesystem would be fine with that and for bcachefs this
> > doesn't work only because they have special superblock instantiation
> > behavior on mount for independent reasons (i.e., not because active ref
> > + s_umount would be problematic for them) if I understand Kent right.
> > So I'm still not fully convinced each multi-device filesystem should be
> > shipping their special method to get from device to stable sb reference.
>
> Honestly, the sync_filesystem() call seems bogus.
>
> If the block device is truly dead, what's it going to accomplish?

Notice that fs_bdev_mark_dead() calls sync_filesystem() only in case
'surprise' argument is false - meaning this is actually a notification
*before* the device is going away. I.e., graceful device hot unplug when
you can access the device to clean up as much as possible.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR