On Mon, Jul 14, 2025 at 02:56:02PM +0930, Qu Wenruo wrote:
> For the ->remove_bdev() callback, btrfs will:
>
> - Mark the target device as missing
>
> - Go degraded if the fs can afford it
>
> - Return error other wise
> Thus falls back to the shutdown callback
>
> For the ->shutdown callback, btrfs will:
>
> - Set the SHUTDOWN flag
> Which will reject all new incoming operations, and make all writeback
> to fail.
>
> The behavior is the same as the NOLOGFLUSH behavior.
>
> To support the lookup from bdev to a btrfs_device,
> btrfs_dev_lookup_args is enhanced to have a new @devt member.
> If set, we should be able to use that @devt member to uniquely locating a
> btrfs device.
>
> I know the shutdown can be a little overkilled, if one has a RAID1
> metadata and RAID0 data, in that case one can still read data with 50%
> chance to got some good data.
>
> But a filesystem returning -EIO for half of the time is not really
> considered usable.
> Further it can also be as bad as the only device went missing for a single
> device btrfs.
>
> So here we go safe other than sorry when handling missing device.
>
> And the remove_bdev callback will be hidden behind experimental features
> for now, the reasons are:
>
> - There are not enough btrfs specific bdev removal test cases
> The existing test cases are all removing the only device, thus only
> exercises the ->shutdown() behavior.
>
> - Not yet determined what's the expected behavior
> Although the current auto-degrade behavior is no worse than the old
> behavior, it may not always be what the end users want.
>
> Before there is a concrete interface, better hide the new feature
> from end users.
>
> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
> ---
> fs/btrfs/super.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++
> fs/btrfs/volumes.c | 2 ++
> fs/btrfs/volumes.h | 5 ++++
> 3 files changed, 73 insertions(+)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 466d0450269c..79f6ad1d44de 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -2413,6 +2413,68 @@ static long btrfs_free_cached_objects(struct super_block *sb, struct shrink_cont
> return 0;
> }
>
> +#ifdef CONFIG_BTRFS_EXPERIMENTAL
> +static int btrfs_remove_bdev(struct super_block *sb, struct block_device *bdev)
> +{
> + struct btrfs_fs_info *fs_info = btrfs_sb(sb);
> + struct btrfs_device *device;
> + struct btrfs_dev_lookup_args lookup_args = { .devt = bdev->bd_dev };
> + bool can_rw;
> +
> + mutex_lock(&fs_info->fs_devices->device_list_mutex);
> + device = btrfs_find_device(fs_info->fs_devices, &lookup_args);
> + if (!device) {
> + mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> + /* Device not found, should not affect the running fs, just give a warning. */
> + btrfs_warn(fs_info, "unable to find btrfs device for block device '%pg'",
> + bdev);
> + return 0;
> + }
I got very confused when reviewing this questioning myself how this is
going to work... until I pulled btrfs/for-next. It would've been good to
know that you merged patches to do bdev_file_open_by_*() calls with the
superblock as owner and fs_holder_ops set. Very good to see this! Thanks
everyone!
So I can now go delete the whole paragraph I had about that. :)
