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

Darrick J. Wong
2025-07-08T20:20:51Z

On Tue, Jul 08, 2025 at 12:20:00PM +0200, Jan Kara wrote:
> On Mon 07-07-25 17:45:32, Darrick J. Wong wrote:
> > On Tue, Jul 08, 2025 at 08:52:47AM +0930, Qu Wenruo wrote:
> > > 在 2025/7/8 08:32, Dave Chinner 写道:
> > > > On Fri, Jul 04, 2025 at 10:12:29AM +0930, Qu Wenruo wrote:
> > > > > Currently all the filesystems implementing the
> > > > > super_opearations::shutdown() callback can not afford losing a device.
> > > > >
> > > > > Thus fs_bdev_mark_dead() will just call the shutdown() callback for the
> > > > > involved filesystem.
> > > > >
> > > > > But it will no longer be the case, with multi-device filesystems like
> > > > > btrfs and bcachefs the filesystem can handle certain device loss without
> > > > > shutting down the whole filesystem.
> > > > >
> > > > > To allow those multi-device filesystems to be integrated to use
> > > > > fs_holder_ops:
> > > > >
> > > > > - Replace super_opearation::shutdown() with
> > > > > super_opearations::remove_bdev()
> > > > > To better describe when the callback is called.
> > > >
> > > > This conflates cause with action.
> > > >
> > > > The shutdown callout is an action that the filesystem must execute,
> > > > whilst "remove bdev" is a cause notification that might require an
> > > > action to be take.
> > > >
> > > > Yes, the cause could be someone doing hot-unplug of the block
> > > > device, but it could also be something going wrong in software
> > > > layers below the filesystem. e.g. dm-thinp having an unrecoverable
> > > > corruption or ENOSPC errors.
> > > >
> > > > We already have a "cause" notification: blk_holder_ops->mark_dead().
> > > >
> > > > The generic fs action that is taken by this notification is
> > > > fs_bdev_mark_dead(). That action is to invalidate caches and shut
> > > > down the filesystem.
> > > >
> > > > btrfs needs to do something different to a blk_holder_ops->mark_dead
> > > > notification. i.e. it needs an action that is different to
> > > > fs_bdev_mark_dead().
> > > >
> > > > Indeed, this is how bcachefs already handles "single device
> > > > died" events for multi-device filesystems - see
> > > > bch2_fs_bdev_mark_dead().
> > >
> > > I do not think it's the correct way to go, especially when there is already
> > > fs_holder_ops.
> > >
> > > We're always going towards a more generic solution, other than letting the
> > > individual fs to do the same thing slightly differently.
> >
> > On second thought -- it's weird that you'd flush the filesystem and
> > shrink the inode/dentry caches in a "your device went away" handler.
> > Fancy filesystems like bcachefs and btrfs would likely just shift IO to
> > a different bdev, right? And there's no good reason to run shrinkers on
> > either of those fses, right?
>
> I agree it is awkward and bcachefs avoids these in case of removal it can
> handle gracefully AFAICS.
>
> > > Yes, the naming is not perfect and mixing cause and action, but the end
> > > result is still a more generic and less duplicated code base.
> >
> > I think dchinner makes a good point that if your filesystem can do
> > something clever on device removal, it should provide its own block
> > device holder ops instead of using fs_holder_ops. I don't understand
> > why you need a "generic" solution for btrfs when it's not going to do
> > what the others do anyway.
>
> Well, I'd also say just go for own fs_holder_ops if it was not for the
> awkward "get super from bdev" step. As Christian wrote we've encapsulated
> that in fs/super.c and bdev_super_lock() in particular but the calling
> conventions for the fs_holder_ops are not very nice (holding
> bdev_holder_lock, need to release it before grabbing practically anything
> else) so I'd have much greater peace of mind if this didn't spread too
> much. Once you call bdev_super_lock() and hold on to sb with s_umount held,
> things are much more conventional for the fs land so I'd like if this
> step happened before any fs hook got called. So I prefer something like
> Qu's proposal of separate sb op for device removal over exporting
> bdev_super_lock(). Like:
>
> static void fs_bdev_mark_dead(struct block_device *bdev, bool surprise)
> {
> struct super_block *sb;
>
> sb = bdev_super_lock(bdev, false);
> if (!sb)
> return;
>
> if (sb->s_op->remove_bdev) {
> sb->s_op->remove_bdev(sb, bdev, surprise);
> return;
> }

It feels odd but I could live with this, particularly since that's the
direction that brauner is laying down. :)

Do we still need to super_unlock_shared here?

--D

>
> if (!surprise)
> sync_filesystem(sb);
> shrink_dcache_sb(sb);
> evict_inodes(sb);
> if (sb->s_op->shutdown)
> sb->s_op->shutdown(sb);
>
> super_unlock_shared(sb);
> }
>
> > As an aside:
> > 'twould be nice if we could lift the *FS_IOC_SHUTDOWN dispatch out of
> > everyone's ioctl functions into the VFS, and then move the "I am dead"
> > state into super_block so that you could actually shut down any
> > filesystem, not just the seven that currently implement it.
>
> Yes, I should find time to revive that patch series... It was not *that*
> hard to do.
>
> Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR
>