Re: [PATCH v2] Btrfs: avoid losing data raid profile when deleting a device

[Liu Bo <bo.li.liu@oracle.com>]

On Wed, Oct 11, 2017 at 10:38:50AM +0300, Nikolay Borisov wrote:
> 
> 
> On 10.10.2017 20:53, Liu Bo wrote:
> > We've avoided data losing raid profile when doing balance, but it
> > turns out that deleting a device could also result in the same
> > problem
> > 
> > This fixes the problem by creating an empty data chunk before
> > relocating the data chunk.
> 
> Why is this needed - copy the metadata of the to-be-relocated chunk into
> the newly created empty chunk? I don't entirely understand that code but
> doesn't this seem a bit like a hack in order to stash some information?
> Perhaps you could elaborate the logic a bit more in the changelog?
> 
> > 
> > Metadata/System chunk are supposed to have non-zero bytes all the time
> > so their raid profile is persistent.
> 
> I think this changelog is a bit scarce on detail as to the culprit of
> the problem. Could you perhaps put a sentence or two what the underlying
> logic which deletes the raid profile if a chunk is empty ?
>

Fair enough.

The problem is as same as what commit 2c9fe8355258 ("btrfs: Fix
lost-data-profile caused by balance bg") had fixed.

Similar to doing balance, deleting a device can also move all chunks
on this disk to other available disks, after 'move' successfully,
it'll remove those chunks.

If our last data chunk is empty and part of it happens to be on this
disk, then there is no data chunk in this btrfs after deleting the
device successfully, any following write will try to create a new data
chunk which ends up with a single data chunk because the only
available data raid profile is 'single'.

thanks,
-liubo

> > 
> > Reported-by: James Alandt <James.Alandt@wdc.com>
> > Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> > ---
> > 
> > v2: - return the correct error.
> >     - move helper ahead of __btrfs_balance().
> > 
> >  fs/btrfs/volumes.c | 84 ++++++++++++++++++++++++++++++++++++++++++------------
> >  1 file changed, 65 insertions(+), 19 deletions(-)
> > 
> > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> > index 4a72c45..a74396d 100644
> > --- a/fs/btrfs/volumes.c
> > +++ b/fs/btrfs/volumes.c
> > @@ -3018,6 +3018,48 @@ static int btrfs_relocate_sys_chunks(struct btrfs_fs_info *fs_info)
> >  	return ret;
> >  }
> >  
> > +/*
> > + * return 1 : allocate a data chunk successfully,
> > + * return <0: errors during allocating a data chunk,
> > + * return 0 : no need to allocate a data chunk.
> > + */
> > +static int btrfs_may_alloc_data_chunk(struct btrfs_fs_info *fs_info,
> > +				      u64 chunk_offset)
> > +{
> > +	struct btrfs_block_group_cache *cache;
> > +	u64 bytes_used;
> > +	u64 chunk_type;
> > +
> > +	cache = btrfs_lookup_block_group(fs_info, chunk_offset);
> > +	ASSERT(cache);
> > +	chunk_type = cache->flags;
> > +	btrfs_put_block_group(cache);
> > +
> > +	if (chunk_type & BTRFS_BLOCK_GROUP_DATA) {
> > +		spin_lock(&fs_info->data_sinfo->lock);
> > +		bytes_used = fs_info->data_sinfo->bytes_used;
> > +		spin_unlock(&fs_info->data_sinfo->lock);
> > +
> > +		if (!bytes_used) {
> > +			struct btrfs_trans_handle *trans;
> > +			int ret;
> > +
> > +			trans =	btrfs_join_transaction(fs_info->tree_root);
> > +			if (IS_ERR(trans))
> > +				return PTR_ERR(trans);
> > +
> > +			ret = btrfs_force_chunk_alloc(trans, fs_info,
> > +						      BTRFS_BLOCK_GROUP_DATA);
> > +			btrfs_end_transaction(trans);
> > +			if (ret < 0)
> > +				return ret;
> > +
> > +			return 1;
> > +		}
> > +	}
> > +	return 0;
> > +}
> > +
> >  static int insert_balance_item(struct btrfs_fs_info *fs_info,
> >  			       struct btrfs_balance_control *bctl)
> >  {
> > @@ -3476,7 +3518,6 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
> >  	u32 count_meta = 0;
> >  	u32 count_sys = 0;
> >  	int chunk_reserved = 0;
> > -	u64 bytes_used = 0;
> >  
> >  	/* step one make some room on all the devices */
> >  	devices = &fs_info->fs_devices->devices;
> > @@ -3635,28 +3676,21 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
> >  			goto loop;
> >  		}
> >  
> > -		ASSERT(fs_info->data_sinfo);
> > -		spin_lock(&fs_info->data_sinfo->lock);
> > -		bytes_used = fs_info->data_sinfo->bytes_used;
> > -		spin_unlock(&fs_info->data_sinfo->lock);
> > -
> > -		if ((chunk_type & BTRFS_BLOCK_GROUP_DATA) &&
> > -		    !chunk_reserved && !bytes_used) {
> > -			trans = btrfs_start_transaction(chunk_root, 0);
> > -			if (IS_ERR(trans)) {
> > -				mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> > -				ret = PTR_ERR(trans);
> > -				goto error;
> > -			}
> > -
> > -			ret = btrfs_force_chunk_alloc(trans, fs_info,
> > -						      BTRFS_BLOCK_GROUP_DATA);
> > -			btrfs_end_transaction(trans);
> > +		if (!chunk_reserved) {
> > +			/*
> > +			 * We may be relocating the only data chunk we have,
> > +			 * which could potentially end up with losing data's
> > +			 * raid profile, so lets allocate an empty one in
> > +			 * advance.
> > +			 */
> > +			ret = btrfs_may_alloc_data_chunk(fs_info,
> > +							 found_key.offset);
> >  			if (ret < 0) {
> >  				mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> >  				goto error;
> > +			} else if (ret == 1) {
> > +				chunk_reserved = 1;
> >  			}
> > -			chunk_reserved = 1;
> >  		}
> >  
> >  		ret = btrfs_relocate_chunk(fs_info, found_key.offset);
> > @@ -4419,6 +4453,18 @@ int btrfs_shrink_device(struct btrfs_device *device, u64 new_size)
> >  		chunk_offset = btrfs_dev_extent_chunk_offset(l, dev_extent);
> >  		btrfs_release_path(path);
> >  
> > +		/*
> > +		 * We may be relocating the only data chunk we have,
> > +		 * which could potentially end up with losing data's
> > +		 * raid profile, so lets allocate an empty one in
> > +		 * advance.
> > +		 */
> > +		ret = btrfs_may_alloc_data_chunk(fs_info, chunk_offset);
> > +		if (ret < 0) {
> > +			mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> > +			goto done;
> > +		}
> > +
> >  		ret = btrfs_relocate_chunk(fs_info, chunk_offset);
> >  		mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> >  		if (ret && ret != -ENOSPC)
> >