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

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

On Mon, Oct 16, 2017 at 11:53:08AM +0300, Nikolay Borisov wrote:
> 
> 
> On 13.10.2017 23:51, Liu Bo wrote:
> > 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
>           ^
> "Last data chunk" of which disk - the "to-be-removed" one or the disk to
> which chunks have been relocated?
>

It refers to the 'to-be-removed' disk.

> > disk, then there is no data chunk in this btrfs after deleting the
>    ^
> Which disk are you referring to - the one being removed?

Yes, the 'to-be-removed' disk.

Thanks,

-liubo

> > 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)
> >>>
> >