Re: [PATCH 2/2] btrfs: scrub: move scrub_setup_ctx allocation out of device_list_mutex

[Nikolay Borisov <nborisov@suse.com>]

On 4.12.18 г. 17:11 ч., David Sterba wrote:
> The scrub context is allocated with GFP_KERNEL and called from
> btrfs_scrub_dev under the fs_info::device_list_mutex. This is not safe
> regarding reclaim that could try to flush filesystem data in order to
> get the memory. And the device_list_mutex is held during superblock
> commit, so this would cause a lockup.
> 
> Move the alocation and initialization before any changes that require
> the mutex.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/scrub.c | 30 ++++++++++++++++++------------
>  1 file changed, 18 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
> index ffcab263e057..051d14c9f013 100644
> --- a/fs/btrfs/scrub.c
> +++ b/fs/btrfs/scrub.c
> @@ -3834,13 +3834,18 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  		return -EINVAL;
>  	}
>  
> +	/* Allocate outside of device_list_mutex */
> +	sctx = scrub_setup_ctx(fs_info, is_dev_replace);
> +	if (IS_ERR(sctx))
> +		return PTR_ERR(sctx);
>  
>  	mutex_lock(&fs_info->fs_devices->device_list_mutex);
>  	dev = btrfs_find_device(fs_info, devid, NULL, NULL);
>  	if (!dev || (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state) &&
>  		     !is_dev_replace)) {
>  		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> -		return -ENODEV;
> +		ret = -ENODEV;
> +		goto out_free_ctx;
>  	}
>  
>  	if (!is_dev_replace && !readonly &&
> @@ -3848,7 +3853,8 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
>  		btrfs_err_in_rcu(fs_info, "scrub: device %s is not writable",
>  				rcu_str_deref(dev->name));
> -		return -EROFS;
> +		ret = -EROFS;
> +		goto out_free_ctx;
>  	}
>  
>  	mutex_lock(&fs_info->scrub_lock);
> @@ -3856,7 +3862,8 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  	    test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &dev->dev_state)) {
>  		mutex_unlock(&fs_info->scrub_lock);
>  		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> -		return -EIO;
> +		ret = -EIO;
> +		goto out_free_ctx;
>  	}
>  
>  	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
> @@ -3866,7 +3873,8 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  		btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
>  		mutex_unlock(&fs_info->scrub_lock);
>  		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> -		return -EINPROGRESS;
> +		ret = -EINPROGRESS;
> +		goto out_free_ctx;
>  	}
>  	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
>  
> @@ -3874,16 +3882,9 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  	if (ret) {
>  		mutex_unlock(&fs_info->scrub_lock);
>  		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> -		return ret;
> +		goto out_free_ctx;

Don't we suffer the same issue when calling scrub_workers_get since in
it we do btrfs_alloc_workqueue which also calls kzalloc with GFP_KERNEL?


>  	}
>  
> -	sctx = scrub_setup_ctx(fs_info, is_dev_replace);
> -	if (IS_ERR(sctx)) {
> -		mutex_unlock(&fs_info->scrub_lock);
> -		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> -		scrub_workers_put(fs_info);
> -		return PTR_ERR(sctx);
> -	}
>  	sctx->readonly = readonly;
>  	dev->scrub_ctx = sctx;
>  	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> @@ -3936,6 +3937,11 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  
>  	scrub_put_ctx(sctx);
>  
> +	return ret;
> +
> +out_free_ctx:
> +	scrub_free_ctx(sctx);
> +
>  	return ret;
>  }
>  
>