Re: [PATCH 1/4] btrfs: delete unused BGs while reclaiming BGs

[Filipe Manana <fdmanana@kernel.org>]

On Tue, Jun 6, 2023 at 7:04 AM Naohiro Aota <naota@elisp.net> wrote:
>
> The reclaiming process only starts after the FS volumes are allocated to a
> certain level (75% by default). Thus, the list of reclaiming target block
> groups can build up so huge at the time the reclaim process kicks in. On a
> test run, there were over 1000 BGs in the reclaim list.
>
> As the reclaim involves rewriting the data, it takes really long time to
> reclaim the BGs. While the reclaim is running, btrfs_delete_unused_bgs()
> won't proceed because the reclaim side is holding
> fs_info->reclaim_bgs_lock. As a result, we will have a large number of unused
> BGs kept in the unused list. On my test run, I got 1057 unused BGs.
>
> Since deleting a block group is relatively easy and fast work, we can call
> btrfs_delete_unused_bgs() while it reclaims BGs, to avoid building up
> unused BGs.
>
> Fixes: 18bb8bbf13c1 ("btrfs: zoned: automatically reclaim zones")
> CC: stable@vger.kernel.org # 5.15+
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Looks good, great catch.

> ---
>  fs/btrfs/block-group.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 618ba7670e66..c5547da0f6eb 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -1824,10 +1824,24 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
>
>  next:
>                 btrfs_put_block_group(bg);
> +
> +               mutex_unlock(&fs_info->reclaim_bgs_lock);
> +               /*
> +                * Reclaiming all the BGs in the list can take really long.
> +                * Prioritize cleanning up unused BGs.
> +                */
> +               btrfs_delete_unused_bgs(fs_info);
> +               /*
> +                * If we are interrupted by a balance, we can just bail out. The
> +                * cleaner thread call me again if necessary.
> +                */
> +               if (!mutex_trylock(&fs_info->reclaim_bgs_lock))
> +                       goto end;
>                 spin_lock(&fs_info->unused_bgs_lock);
>         }
>         spin_unlock(&fs_info->unused_bgs_lock);
>         mutex_unlock(&fs_info->reclaim_bgs_lock);
> +end:
>         btrfs_exclop_finish(fs_info);
>         sb_end_write(fs_info->sb);
>  }
> --
> 2.40.1
>