Re: [PATCH 3/4] xfs: xfs_defer_capture should absorb remaining block reservation

[Dave Chinner <david@fromorbit.com>]

On Sun, Sep 27, 2020 at 04:41:27PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> When xfs_defer_capture extracts the deferred ops and transaction state
> from a transaction, it should absorb the remaining block reservation so
> that when we continue the dfops chain, we still have those blocks to
> use.
> 
> This adds the requirement that every log intent item recovery function
> must be careful to reserve enough blocks to handle both itself and all
> defer ops that it can queue.  On the other hand, this enables us to do
> away with the handwaving block estimation nonsense that was going on in
> xlog_finish_defer_ops.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/libxfs/xfs_defer.c |    4 ++++
>  fs/xfs/libxfs/xfs_defer.h |    1 +
>  fs/xfs/xfs_log_recover.c  |   18 +-----------------
>  3 files changed, 6 insertions(+), 17 deletions(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
> index 0de7672fe63d..85d70f1edc1c 100644
> --- a/fs/xfs/libxfs/xfs_defer.c
> +++ b/fs/xfs/libxfs/xfs_defer.c
> @@ -574,6 +574,8 @@ xfs_defer_capture(
>  	list_splice_init(&tp->t_dfops, &dfc->dfc_dfops);
>  	dfc->dfc_tpflags = tp->t_flags & XFS_TRANS_LOWMODE;
>  	tp->t_flags &= ~XFS_TRANS_LOWMODE;
> +	dfc->dfc_blkres = tp->t_blk_res - tp->t_blk_res_used;
> +	tp->t_blk_res = tp->t_blk_res_used;
>  
>  	return dfc;
>  }

I think this needs an explanation in the function/API comment to
explain why it is being done.

Otherwise looks good.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com