linux-ext4 - Re: [PATCH v4 1/6] fs: enhance and rename shutdown() callback to remove

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Message-ID: <kgolzhhd47x3iqkdrwyzh65ng4mm6cauxdjgiao2otztncyc3f@rskadwaph2l5>
Date: Thu, 10 Jul 2025 15:10:04 +0200
From: Jan Kara <jack@...e.cz>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>, 
	Christian Brauner <brauner@...nel.org>, "Darrick J. Wong" <djwong@...nel.org>, 
	Qu Wenruo <quwenruo.btrfs@....com>, Qu Wenruo <wqu@...e.com>, linux-btrfs@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk, linux-ext4@...r.kernel.org, 
	linux-f2fs-devel@...ts.sourceforge.net, ntfs3@...ts.linux.dev, linux-xfs@...r.kernel.org, 
	linux-bcachefs@...r.kernel.org
Subject: Re: [PATCH v4 1/6] fs: enhance and rename shutdown() callback to
 remove_bdev()

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@...e.com>
SUSE Labs, CR