Re: [PATCH 2/6] btrfs: zoned: add a dedicated data relocation block group

[Naohiro Aota <Naohiro.Aota@wdc.com>]

On Fri, Sep 03, 2021 at 11:44:43PM +0900, Johannes Thumshirn wrote:
> Relocation in a zoned filesystem can fail with a transaction abort with
> error -22 (EINVAL). This happens because the relocation code assumes that
> the extents we relocated the data to have the same size the source extents
> had and ensures this by preallocating the extents.
> 
> But in a zoned filesystem we currently can't preallocate the extents as
> this would break the sequential write required rule. Therefore it can
> happen that the writeback process kicks in while we're still adding pages
> to a delallocation range and starts writing out dirty pages.
> 
> This then creates destination extents that are smaller than the source
> extents, triggering the following safety check in get_new_location():
> 
>  1034         if (num_bytes != btrfs_file_extent_disk_num_bytes(leaf, fi)) {
>  1035                 ret = -EINVAL;
>  1036                 goto out;
>  1037         }
> 
> Temporarily create a dedicated block group for the relocation process, so
> no non-relocation data writes can interfere with the relocation writes.
> 
> This is needed that we can switch the relocation process on a zoned
> filesystem from the REQ_OP_ZONE_APPEND writing we use for data to a scheme
> like in a non-zoned filesystem using REQ_OP_WRITE and preallocation.
> 
> Fixes: 32430c614844 ("btrfs: zoned: enable relocation on a zoned filesystem")
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---

Looks good,
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>