Re: [PATCH 2/2] btrfs: introduce feature to ignore a btrfs device

[Nikolay Borisov <nborisov@suse.com>]

On 13.11.2017 07:44, Anand Jain wrote:
> Support for a new command is being added here:
>  btrfs dev ignore <dev>
> Which shall undo the effects of the command
>  btrfs dev scan <dev>
> 
> This cli/ioctl is needed as there is no way to continue to mount in
> degraded mode if the device is already scanned, which is required to
> recover from the split brain raid conditions.
> 
> This patch proposes to use ioctl #5 as it was empty.
> 	IOW(BTRFS_IOCTL_MAGIC, 5, ..)
> If #5 is reserved for some other purpose, I think I should change this.
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>  fs/btrfs/super.c           |  4 +++
>  fs/btrfs/volumes.c         | 70 ++++++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/volumes.h         |  2 ++
>  include/uapi/linux/btrfs.h |  2 ++
>  4 files changed, 78 insertions(+)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index fa7bad8b22da..64f4beb1526d 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -2183,6 +2183,10 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd,
>  		ret = btrfs_scan_one_device(vol->name, FMODE_READ,
>  					    &btrfs_fs_type, &fs_devices);
>  		break;
> +	case BTRFS_IOC_IGNORE_DEV:
> +		ret = btrfs_ignore_one_device(vol->name, FMODE_READ,
> +					    &btrfs_fs_type, &fs_devices);
> +		break;
>  	case BTRFS_IOC_DEVICES_READY:
>  		ret = btrfs_scan_one_device(vol->name, FMODE_READ,
>  					    &btrfs_fs_type, &fs_devices);
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 8ead85dba6f5..1a06a17e111e 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1181,6 +1181,76 @@ int btrfs_read_disk_super(struct block_device *bdev, u64 bytenr,
>  	return 0;
>  }
>  
> +static int device_list_remove(struct btrfs_super_block *disk_super, u64 devid)
> +{
> +	int ret = 0;
> +	struct btrfs_fs_devices *fs_devices;
> +	struct btrfs_device *device;
> +
> +	fs_devices = find_fsid(disk_super->fsid);
> +	if (!fs_devices)
> +		return -ENOENT;
> +
> +	if (fs_devices->opened)
> +		return -EPERM;

Isn't -EBUSY more appropriate?

> +
> +	mutex_lock(&uuid_mutex);
> +	mutex_lock(&fs_devices->device_list_mutex);
> +	device = find_device(fs_devices, devid, disk_super->dev_item.uuid);
> +	if (!device) {
> +		ret = -ENOENT;
> +		goto error;
> +	}
> +
> +	delete_device_from_list(device);
> +
> +error:
> +	mutex_unlock(&fs_devices->device_list_mutex);
> +	mutex_unlock(&uuid_mutex);
> +
> +	return ret;
> +}
> +
> +int btrfs_ignore_one_device(const char *path, fmode_t flags, void *holder,
> +			  struct btrfs_fs_devices **fs_devices_ret)
> +{
> +	struct btrfs_super_block *disk_super;
> +	struct block_device *bdev;
> +	struct page *page;
> +	int ret = -EINVAL;
> +	u64 devid;
> +	u64 bytenr;
> +
> +	bytenr = btrfs_sb_offset(0);
> +	flags |= FMODE_EXCL;
> +
> +	bdev = blkdev_get_by_path(path, flags, holder);
> +	if (IS_ERR(bdev)) {
> +		ret = PTR_ERR(bdev);
> +		goto error;
> +	}
> +
> +	if (btrfs_read_disk_super(bdev, bytenr, &page, &disk_super))
> +		goto error_bdev_put;
> +
> +	devid = btrfs_stack_device_id(&disk_super->dev_item);
> +
> +	ret = device_list_remove(disk_super, devid);
> +	if (ret)
> +	pr_err("BTRFS: %pU: device %s uuid %pU devid %llu failed to unscan: %d",
> +		disk_super->fsid, path, disk_super->dev_item.uuid, devid, ret);
> +	else
> +	pr_info("BTRFS: %pU: device %s uuid %pU devid %llu ran undo scan",
> +		disk_super->fsid, path, disk_super->dev_item.uuid, devid);
> +
> +	btrfs_release_disk_super(page);
> +
> +error_bdev_put:
> +	blkdev_put(bdev, flags);
> +error:
> +	return ret;
> +}
> +
>  /*
>   * Look for a btrfs signature on a device. This may be called out of the mount path
>   * and we are not allowed to call set_blocksize during the scan. The superblock
> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> index 6108fdfec67f..340e40acdab5 100644
> --- a/fs/btrfs/volumes.h
> +++ b/fs/btrfs/volumes.h
> @@ -421,6 +421,8 @@ int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
>  		       fmode_t flags, void *holder);
>  int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
>  			  struct btrfs_fs_devices **fs_devices_ret);
> +int btrfs_ignore_one_device(const char *path, fmode_t flags, void *holder,
> +			  struct btrfs_fs_devices **fs_devices_ret);
>  int btrfs_close_devices(struct btrfs_fs_devices *fs_devices);
>  void btrfs_close_extra_devices(struct btrfs_fs_devices *fs_devices, int step);
>  void btrfs_assign_next_active_device(struct btrfs_fs_info *fs_info,
> diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
> index 378230c163d5..928485c31aa4 100644
> --- a/include/uapi/linux/btrfs.h
> +++ b/include/uapi/linux/btrfs.h
> @@ -739,6 +739,8 @@ enum btrfs_err_code {
>  				   struct btrfs_ioctl_vol_args)
>  #define BTRFS_IOC_SCAN_DEV _IOW(BTRFS_IOCTL_MAGIC, 4, \
>  				   struct btrfs_ioctl_vol_args)
> +#define BTRFS_IOC_IGNORE_DEV _IOW(BTRFS_IOCTL_MAGIC, 5, \
> +				   struct btrfs_ioctl_vol_args)
>  /* trans start and trans end are dangerous, and only for
>   * use by applications that know how to avoid the
>   * resulting deadlocks
>