Re: [PATCH v3 1/5] btrfs: replace stripe extents

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Jul 01, 2024 at 12:25:15PM +0200, Johannes Thumshirn wrote:
> From: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> 
> If we can't insert a stripe extent in the RAID stripe tree, because
> the key that points to the specific position in the stripe tree is
> already existing, we have to remove the item and then replace it by a
> new item.
> 
> This can happen for example on device replace operations.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
>  fs/btrfs/raid-stripe-tree.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/fs/btrfs/raid-stripe-tree.c b/fs/btrfs/raid-stripe-tree.c
> index e6f7a234b8f6..3020820dd6e2 100644
> --- a/fs/btrfs/raid-stripe-tree.c
> +++ b/fs/btrfs/raid-stripe-tree.c
> @@ -73,6 +73,37 @@ int btrfs_delete_raid_extent(struct btrfs_trans_handle *trans, u64 start, u64 le
>  	return ret;
>  }
>  
> +static int replace_raid_extent_item(struct btrfs_trans_handle *trans,
> +				    struct btrfs_key *key,
> +				    struct btrfs_stripe_extent *stripe_extent,
> +				    const size_t item_size)
> +{
> +	struct btrfs_fs_info *fs_info = trans->fs_info;
> +	struct btrfs_root *stripe_root = fs_info->stripe_root;
> +	struct btrfs_path *path;
> +	int ret;
> +
> +	path = btrfs_alloc_path();
> +	if (!path)
> +		return -ENOMEM;
> +
> +	ret = btrfs_search_slot(trans, stripe_root, key, path, -1, 1);
> +	if (ret)
> +		goto err;
> +
> +	ret = btrfs_del_item(trans, stripe_root, path);
> +	if (ret)
> +		goto err;
> +
> +	btrfs_free_path(path);
> +
> +	return btrfs_insert_item(trans, stripe_root, key, stripe_extent,
> +				 item_size);
> + err:
> +	btrfs_free_path(path);
> +	return ret;
> +}
> +
>  static int btrfs_insert_one_raid_extent(struct btrfs_trans_handle *trans,
>  					struct btrfs_io_context *bioc)
>  {
> @@ -112,6 +143,9 @@ static int btrfs_insert_one_raid_extent(struct btrfs_trans_handle *trans,
>  
>  	ret = btrfs_insert_item(trans, stripe_root, &stripe_key, stripe_extent,
>  				item_size);
> +	if (ret == -EEXIST)
> +		ret = replace_raid_extent_item(trans, &stripe_key,
> +					       stripe_extent, item_size);

I had another thought, how often is this particular thing happening?  Bec ause
we're doing 3 path allocations here in the worst case.  If it happens more than
say 10% of the time then we need to allocate a path once in
btrfs_insert_one_raid_extent(), do the insert, and if it fails re-use that path
to do the delete and insert the new one.  Thanks,

Josef