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

Kent Overstreet
2025-07-10T18:41:25Z

On Thu, Jul 10, 2025 at 03:10:04PM +0200, Jan Kara wrote:
> 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.

That doesn't seem to be hooked up to anything?

blk_mark_disk_dead() -> blk_report_disk_dead(), surprise is always true

disk_force_media_change(), same

The only call where it's falso is in s390 code. If we know that a disk
is going away, that would be a userspace thing, and they can just
unmount.