Re: [PATCH v6 6/7] xfs: support shrinking unused space in the last AG

[Brian Foster <bfoster@redhat.com>]

On Tue, Jan 26, 2021 at 08:56:20PM +0800, Gao Xiang wrote:
> As the first step of shrinking, this attempts to enable shrinking
> unused space in the last allocation group by fixing up freespace
> btree, agi, agf and adjusting super block and use a helper
> xfs_ag_shrink_space() to fixup the last AG.
> 
> This can be all done in one transaction for now, so I think no
> additional protection is needed.
> 
> Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
> ---
>  fs/xfs/xfs_fsops.c | 64 ++++++++++++++++++++++++++++++----------------
>  fs/xfs/xfs_trans.c |  1 -
>  2 files changed, 42 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
> index 6c4ab5e31054..4bcea22f7b3f 100644
> --- a/fs/xfs/xfs_fsops.c
> +++ b/fs/xfs/xfs_fsops.c
> @@ -38,7 +38,7 @@ xfs_resizefs_init_new_ags(
>  	struct aghdr_init_data	*id,
>  	xfs_agnumber_t		oagcount,
>  	xfs_agnumber_t		nagcount,
> -	xfs_rfsblock_t		*delta)
> +	int64_t			*delta)
>  {
>  	xfs_rfsblock_t		nb = mp->m_sb.sb_dblocks + *delta;
>  	int			error;
> @@ -76,33 +76,41 @@ xfs_growfs_data_private(
>  	xfs_agnumber_t		nagcount;
>  	xfs_agnumber_t		nagimax = 0;
>  	xfs_rfsblock_t		nb, nb_div, nb_mod;
> -	xfs_rfsblock_t		delta;
> +	int64_t			delta;
>  	xfs_agnumber_t		oagcount;
>  	struct xfs_trans	*tp;
> +	bool			extend;
>  	struct aghdr_init_data	id = {};
>  
>  	nb = in->newblocks;
> -	if (nb < mp->m_sb.sb_dblocks)
> -		return -EINVAL;
> -	if ((error = xfs_sb_validate_fsb_count(&mp->m_sb, nb)))
> +	if (nb == mp->m_sb.sb_dblocks)
> +		return 0;
> +
> +	error = xfs_sb_validate_fsb_count(&mp->m_sb, nb);
> +	if (error)
>  		return error;
> -	error = xfs_buf_read_uncached(mp->m_ddev_targp,
> +
> +	if (nb > mp->m_sb.sb_dblocks) {
> +		error = xfs_buf_read_uncached(mp->m_ddev_targp,
>  				XFS_FSB_TO_BB(mp, nb) - XFS_FSS_TO_BB(mp, 1),
>  				XFS_FSS_TO_BB(mp, 1), 0, &bp, NULL);
> -	if (error)
> -		return error;
> -	xfs_buf_relse(bp);
> +		if (error)
> +			return error;
> +		xfs_buf_relse(bp);
> +	}
>  
>  	nb_div = nb;
>  	nb_mod = do_div(nb_div, mp->m_sb.sb_agblocks);
>  	nagcount = nb_div + (nb_mod != 0);
>  	if (nb_mod && nb_mod < XFS_MIN_AG_BLOCKS) {
>  		nagcount--;
> -		nb = (xfs_rfsblock_t)nagcount * mp->m_sb.sb_agblocks;
> -		if (nb < mp->m_sb.sb_dblocks)
> +		if (nagcount < 2)
>  			return -EINVAL;

What's the reason for the nagcount < 2 check? IIRC we warn about this
configuration at mkfs time, but allow it to proceed. Is it just that we
don't want to accidentally put the fs into an agcount == 1 state that
was originally formatted with >1 AGs?

What about the case where we attempt to grow an agcount == 1 fs but
don't enlarge enough to add the second AG? Does this change error
behavior in that case?

> +		nb = (xfs_rfsblock_t)nagcount * mp->m_sb.sb_agblocks;
>  	}
> +
>  	delta = nb - mp->m_sb.sb_dblocks;
> +	extend = (delta > 0);
>  	oagcount = mp->m_sb.sb_agcount;
>  
>  	/* allocate the new per-ag structures */
> @@ -110,22 +118,34 @@ xfs_growfs_data_private(
>  		error = xfs_initialize_perag(mp, nagcount, &nagimax);
>  		if (error)
>  			return error;
> +	} else if (nagcount < oagcount) {
> +		/* TODO: shrinking the entire AGs hasn't yet completed */
> +		return -EINVAL;
>  	}
>  
>  	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_growdata,
> -			XFS_GROWFS_SPACE_RES(mp), 0, XFS_TRANS_RESERVE, &tp);
> +			(extend ? XFS_GROWFS_SPACE_RES(mp) : -delta), 0,
> +			XFS_TRANS_RESERVE, &tp);
>  	if (error)
>  		return error;
>  
> -	error = xfs_resizefs_init_new_ags(mp, &id, oagcount, nagcount, &delta);
> -	if (error)
> -		goto out_trans_cancel;
> -
> +	if (extend) {
> +		error = xfs_resizefs_init_new_ags(mp, &id, oagcount,
> +						  nagcount, &delta);
> +		if (error)
> +			goto out_trans_cancel;
> +	}
>  	xfs_trans_agblocks_delta(tp, id.nfree);

It looks like id isn't used until the resize call above. Is this call
relevant for the shrink case?

>  
> -	/* If there are new blocks in the old last AG, extend it. */
> +	/* If there are some blocks in the last AG, resize it. */
>  	if (delta) {

This patch added a (nb == mp->m_sb.sb_dblocks) shortcut check at the top
of the function. Should we ever get to this point with delta == 0? (If
not, maybe convert it to an assert just to be safe.)

> -		error = xfs_ag_extend_space(mp, tp, &id, delta);
> +		if (extend) {
> +			error = xfs_ag_extend_space(mp, tp, &id, delta);
> +		} else {
> +			id.agno = nagcount - 1;
> +			error = xfs_ag_shrink_space(mp, &tp, &id, -delta);

xfs_ag_shrink_space() looks like it only accesses id->agno. Perhaps just
pass in agno for now..?

> +		}
> +
>  		if (error)
>  			goto out_trans_cancel;
>  	}
> @@ -137,15 +157,15 @@ xfs_growfs_data_private(
>  	 */
>  	if (nagcount > oagcount)
>  		xfs_trans_mod_sb(tp, XFS_TRANS_SB_AGCOUNT, nagcount - oagcount);
> -	if (nb > mp->m_sb.sb_dblocks)
> +	if (nb != mp->m_sb.sb_dblocks)
>  		xfs_trans_mod_sb(tp, XFS_TRANS_SB_DBLOCKS,
>  				 nb - mp->m_sb.sb_dblocks);

Maybe use delta here?

>  	if (id.nfree)
>  		xfs_trans_mod_sb(tp, XFS_TRANS_SB_FDBLOCKS, id.nfree);
>  

id.nfree tracks newly added free space in the growfs space. Is it not
used in the shrink case because the allocation handles this for us?

>  	/*
> -	 * update in-core counters now to reflect the real numbers
> -	 * (especially sb_fdblocks)
> +	 * update in-core counters now to reflect the real numbers (especially
> +	 * sb_fdblocks). And xfs_validate_sb_write() can pass for shrinkfs.
>  	 */
>  	if (xfs_sb_version_haslazysbcount(&mp->m_sb))
>  		xfs_log_sb(tp);
> @@ -165,7 +185,7 @@ xfs_growfs_data_private(
>  	 * If we expanded the last AG, free the per-AG reservation
>  	 * so we can reinitialize it with the new size.
>  	 */
> -	if (delta) {
> +	if (extend && delta) {
>  		struct xfs_perag	*pag;
>  
>  		pag = xfs_perag_get(mp, id.agno);

We call xfs_fs_reserve_ag_blocks() a bit further down before we exit
this function. xfs_ag_shrink_space() from the previous patch is intended
to deal with perag reservation changes for shrink, but it looks like the
reserve call further down could potentially reset mp->m_finobt_nores to
false if it previously might have been set to true.

Brian

> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index e72730f85af1..fd2cbf414b80 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -419,7 +419,6 @@ xfs_trans_mod_sb(
>  		tp->t_res_frextents_delta += delta;
>  		break;
>  	case XFS_TRANS_SB_DBLOCKS:
> -		ASSERT(delta > 0);
>  		tp->t_dblocks_delta += delta;
>  		break;
>  	case XFS_TRANS_SB_AGCOUNT:
> -- 
> 2.27.0
>