Re: [PATCH 6/8] btrfs: loop in inode_rsv_refill

[Nikolay Borisov <nborisov@suse.com>]

On 3.12.18 г. 17:24 ч., Josef Bacik wrote:
> With severe fragmentation we can end up with our inode rsv size being
> huge during writeout, which would cause us to need to make very large
> metadata reservations.  However we may not actually need that much once

The sentence beginning with "However" needs more information, why might
we not need that much once writeout is complete?
> writeout is complete.  So instead try to make our reservation, and if we
> couldn't make it re-calculate our new reservation size and try again.

Why do you think that recalculating the requested bytes will be
different the 2nd time ?

> If our reservation size doesn't change between tries then we know we are
> actually out of space and can error out.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/extent-tree.c | 58 +++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 43 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 0ee77a98f867..0e1a499035ac 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -5787,6 +5787,21 @@ int btrfs_block_rsv_refill(struct btrfs_root *root,
>  	return ret;
>  }
>  
> +static inline void __get_refill_bytes(struct btrfs_block_rsv *block_rsv,
> +				      u64 *metadata_bytes, u64 *qgroup_bytes)

This function needs a better name. Something like calc_required_bytes or
calc_refill_bytes

> +{
> +	*metadata_bytes = 0;
> +	*qgroup_bytes = 0;
> +
> +	spin_lock(&block_rsv->lock);
> +	if (block_rsv->reserved < block_rsv->size)
> +		*metadata_bytes = block_rsv->size - block_rsv->reserved;
> +	if (block_rsv->qgroup_rsv_reserved < block_rsv->qgroup_rsv_size)
> +		*qgroup_bytes = block_rsv->qgroup_rsv_size -
> +			block_rsv->qgroup_rsv_reserved;
> +	spin_unlock(&block_rsv->lock);
> +}
> +
>  /**
>   * btrfs_inode_rsv_refill - refill the inode block rsv.
>   * @inode - the inode we are refilling.
> @@ -5802,25 +5817,39 @@ static int btrfs_inode_rsv_refill(struct btrfs_inode *inode,
>  {
>  	struct btrfs_root *root = inode->root;
>  	struct btrfs_block_rsv *block_rsv = &inode->block_rsv;
> -	u64 num_bytes = 0;
> +	u64 num_bytes = 0, last = 0;
>  	u64 qgroup_num_bytes = 0;
>  	int ret = -ENOSPC;
>  
> -	spin_lock(&block_rsv->lock);
> -	if (block_rsv->reserved < block_rsv->size)
> -		num_bytes = block_rsv->size - block_rsv->reserved;
> -	if (block_rsv->qgroup_rsv_reserved < block_rsv->qgroup_rsv_size)
> -		qgroup_num_bytes = block_rsv->qgroup_rsv_size -
> -				   block_rsv->qgroup_rsv_reserved;
> -	spin_unlock(&block_rsv->lock);
> -
> +	__get_refill_bytes(block_rsv, &num_bytes, &qgroup_num_bytes);
>  	if (num_bytes == 0)
>  		return 0;
>  
> -	ret = btrfs_qgroup_reserve_meta_prealloc(root, qgroup_num_bytes, true);
> -	if (ret)
> -		return ret;
> -	ret = reserve_metadata_bytes(root, block_rsv, num_bytes, flush);
> +	do {
> +		ret = btrfs_qgroup_reserve_meta_prealloc(root, qgroup_num_bytes, true);
> +		if (ret)
> +			return ret;
> +		ret = reserve_metadata_bytes(root, block_rsv, num_bytes, flush);
> +		if (ret) {
> +			btrfs_qgroup_free_meta_prealloc(root, qgroup_num_bytes);
> +			last = num_bytes;
> +			/*
> +			 * If we are fragmented we can end up with a lot of
> +			 * outstanding extents which will make our size be much
> +			 * larger than our reserved amount.  If we happen to
> +			 * try to do a reservation here that may result in us
> +			 * trying to do a pretty hefty reservation, which we may
> +			 * not need once delalloc flushing happens.  If this is

The "If we happen" sentence needs to be reworded because it's -ENOPARSE.
Perhaps one of the "to do a reservation" should go away?

> +			 * the case try and do the reserve again.
> +			 */
> +			if (flush == BTRFS_RESERVE_FLUSH_ALL)
> +				__get_refill_bytes(block_rsv, &num_bytes,
> +						   &qgroup_num_bytes);
> +			if (num_bytes == 0)
> +				return 0;
> +		}
> +	} while (ret && last != num_bytes);
> +
>  	if (!ret) {
>  		block_rsv_add_bytes(block_rsv, num_bytes, false);
>  		trace_btrfs_space_reservation(root->fs_info, "delalloc",
> @@ -5830,8 +5859,7 @@ static int btrfs_inode_rsv_refill(struct btrfs_inode *inode,
>  		spin_lock(&block_rsv->lock);
>  		block_rsv->qgroup_rsv_reserved += qgroup_num_bytes;
>  		spin_unlock(&block_rsv->lock);
> -	} else
> -		btrfs_qgroup_free_meta_prealloc(root, qgroup_num_bytes);
> +	}
>  	return ret;
>  }
>  
>