Re: [PATCH RFC 5/6] fs: introduce a shutdown_bdev super block operation

[Qu Wenruo <wqu@suse.com>]

在 2025/6/24 18:21, Christian Brauner 写道:
> On Tue, Jun 24, 2025 at 06:57:08AM +0930, Qu Wenruo wrote:
>>
>>
>> 在 2025/6/23 23:27, Christoph Hellwig 写道:
>>> On Mon, Jun 23, 2025 at 12:56:28PM +0200, Christian Brauner wrote:
>>>>           void (*shutdown)(struct super_block *sb);
>>>> +       void (*drop_bdev)(struct super_block *sb, struct block_device *bdev /* , unsigned int flags/reason maybe too ? */);
>>>>    };
>>>>
>>>> You might want to drop a block device independent of whether the device
>>>> was somehow lost. So I find that a bit more flexible.
>>>
>>> Drop is weird word for what is happening here, and if it wasn't for the
>>> context in this thread I'd expect it to be about refcounting in Linux.
>>>
>>> When the VFS/libfs does an upcall into the file system to notify it
>>> that a device is gone that's pretty much a device loss.  I'm not married
>>> to the exact name, but drop seems like a pretty bad choice.
>>
>> What about a more common used term, mark_dead()?
>>
>> It's already used in blk_holder_ops, and I'd say it's more straighforward to
>> me, compared to shutdown()/goingdown().
> 
> But it's not about the superblock going down necessarily. It's about the
> device going away for whatever reason:
> 
> void (*yank_bdev)(struct super_block *sb, struct block_device *bdev /* , unsigned int flags/reason maybe too ? */);
> void (*pull_bdev)(struct super_block *sb, struct block_device *bdev /* , unsigned int flags/reason maybe too ? */);
> void (*unplug_bdev)(struct super_block *sb, struct block_device *bdev /* , unsigned int flags/reason maybe too ? */);
> void (*remove_bdev)(struct super_block *sb, struct block_device *bdev /* , unsigned int flags/reason maybe too ? */);

All sound good to me, although the last one sounds better.

> 
> On a single device superblock unplugging that device would obviously
> cause an actual shutdown. On multi-device superblocks it doesn't always.
> 
> (That brings me to another thought. Is there a use-case for knowing in
> advance whether removing a device would shut down the superblock?

Maybe another interface like can_remove_bdev()?

It's not hard for btrfs to provide it, we already have a check function 
btrfs_check_rw_degradable() to do that.

Although I'd say, that will be something way down the road.

We even don't have a proper way to let end user configure the device 
loss behavior.
E.g. some end users may prefer a full shutdown to be extra cautious, 
other than continue degraded.

Thanks,
Qu

> Because then the ability to probe whether a device can be safely
> removed or an option to only remove the device if it can be removed
> without killing the superblock would be a natural extension.)