Re: [PATCH v4 3/5] btrfs: introduce BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN

[David Sterba <dsterba@suse.cz>]

On Wed, Aug 24, 2022 at 09:14:07AM +0800, Qu Wenruo wrote:
> Here we introduce a new runtime flag,
> BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN, which will inform qgroup rescan
> to cancel its work asynchronously.
> 
> This is to address the window when an operation makes qgroup numbers
> inconsistent (like qgroup inheriting) while a qgroup rescan is running.
> 
> In that case, qgroup inconsistent flag will be cleared when qgroup
> rescan finishes.
> But we changed the ownership of some extents, which means the rescan is
> already meaningless, and the qgroup inconsistent flag should not be
> cleared.
> 
> With the new flag, each time we set INCONSISTENT flag, we also set this
> new flag to inform any running qgroup rescan to exit immediately, and
> leaving the INCONSISTENT flag there.
> 
> The new runtime flag can only be cleared when a new rescan is started.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/qgroup.c | 46 +++++++++++++++++++++++++++++-----------------
>  fs/btrfs/qgroup.h |  2 ++
>  2 files changed, 31 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index fc2ed19ced9b..1b416ac9c3ad 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -333,6 +333,14 @@ int btrfs_verify_qgroup_counts(struct btrfs_fs_info *fs_info, u64 qgroupid,
>  }
>  #endif
>  
> +static void qgroup_mark_inconsistent(struct btrfs_fs_info *fs_info)
> +{
> +	BUILD_BUG_ON(BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN &
> +		     BTRFS_QGROUP_STATUS_FLAGS_MASK);

Please use static_assert instead of BUILD_BUG_ON

> +	fs_info->qgroup_flags |= (BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT |
> +				  BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN);

There should be either a helper to drop the inconsistent flag again or
qgroup_mark_inconsistent can take a parameter to do so, there are
several instances where unsetting is open coded by the "&= ~" operation.