Re: [PATCH 3/3] xfs: use deferred frees to reap old btree blocks

[Brian Foster <bfoster@redhat.com>]

On Wed, Oct 09, 2019 at 09:48:53AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Use deferred frees (EFIs) to reap the blocks of a btree that we just
> replaced.  This helps us to shrink the window in which those old blocks
> could be lost due to a system crash, though we try to flush the EFIs
> every few hundred blocks so that we don't also overflow the transaction
> reservations during and after we commit the new btree.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/scrub/repair.c |   29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)
> 
> 
> diff --git a/fs/xfs/scrub/repair.c b/fs/xfs/scrub/repair.c
> index e21faef6db5a..8349694f985d 100644
> --- a/fs/xfs/scrub/repair.c
> +++ b/fs/xfs/scrub/repair.c
...
> @@ -565,14 +568,24 @@ xrep_reap_block(
>  		xrep_reap_invalidate_block(sc, fsbno);
>  		error = xrep_put_freelist(sc, agbno);
>  	} else {
> +		/*
> +		 * Use deferred frees to get rid of the old btree blocks to try
> +		 * to minimize the window in which we could crash and lose the
> +		 * old blocks.  However, we still need to roll the transaction
> +		 * every 100 or so EFIs so that we don't exceed the log
> +		 * reservation.
> +		 */
>  		xrep_reap_invalidate_block(sc, fsbno);
> -		error = xfs_free_extent(sc->tp, fsbno, 1, oinfo, resv);
> +		__xfs_bmap_add_free(sc->tp, fsbno, 1, oinfo, true);

xfs_free_extent() sets skip_discard to false and this changes it to
true. Intentional?

Otherwise the rest looks straightforward.

Brian

> +		(*deferred)++;
> +		need_roll = *deferred > 100;
>  	}
>  	if (agf_bp != sc->sa.agf_bp)
>  		xfs_trans_brelse(sc->tp, agf_bp);
> -	if (error)
> +	if (error || !need_roll)
>  		return error;
>  
> +	*deferred = 0;
>  	if (sc->ip)
>  		return xfs_trans_roll_inode(&sc->tp, sc->ip);
>  	return xrep_roll_ag_trans(sc);
> @@ -594,6 +607,7 @@ xrep_reap_extents(
>  	struct xfs_bitmap_range		*bmr;
>  	struct xfs_bitmap_range		*n;
>  	xfs_fsblock_t			fsbno;
> +	unsigned int			deferred = 0;
>  	int				error = 0;
>  
>  	ASSERT(xfs_sb_version_hasrmapbt(&sc->mp->m_sb));
> @@ -605,12 +619,17 @@ xrep_reap_extents(
>  				XFS_FSB_TO_AGNO(sc->mp, fsbno),
>  				XFS_FSB_TO_AGBNO(sc->mp, fsbno), 1);
>  
> -		error = xrep_reap_block(sc, fsbno, oinfo, type);
> +		error = xrep_reap_block(sc, fsbno, oinfo, type, &deferred);
>  		if (error)
>  			break;
>  	}
>  
> -	return error;
> +	if (error || deferred == 0)
> +		return error;
> +
> +	if (sc->ip)
> +		return xfs_trans_roll_inode(&sc->tp, sc->ip);
> +	return xrep_roll_ag_trans(sc);
>  }
>  
>  /*
>