Re: [PATCH v4 4/4] xfs: support shrinking unused space in the last AG

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jan 11, 2021 at 09:22:43PM +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 introduce 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/libxfs/xfs_ag.c | 72 ++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/libxfs/xfs_ag.h |  2 ++
>  fs/xfs/xfs_fsops.c     | 70 +++++++++++++++++++++++++++++-----------
>  fs/xfs/xfs_trans.c     |  1 -
>  4 files changed, 125 insertions(+), 20 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
> index 9331f3516afa..bec10c85e2a9 100644
> --- a/fs/xfs/libxfs/xfs_ag.c
> +++ b/fs/xfs/libxfs/xfs_ag.c
> @@ -485,6 +485,78 @@ xfs_ag_init_headers(
>  	return error;
>  }
>  
> +int
> +xfs_ag_shrink_space(
> +	struct xfs_mount	*mp,
> +	struct xfs_trans	*tp,
> +	struct aghdr_init_data	*id,
> +	xfs_extlen_t		len)
> +{
> +	struct xfs_buf		*agibp, *agfbp;
> +	struct xfs_agi		*agi;
> +	struct xfs_agf		*agf;
> +	int			error, err2;
> +	struct xfs_alloc_arg	args = {
> +		.tp	= tp,
> +		.mp	= mp,
> +		.type	= XFS_ALLOCTYPE_THIS_BNO,
> +		.minlen = len,
> +		.maxlen = len,
> +		.oinfo	= XFS_RMAP_OINFO_SKIP_UPDATE,
> +		.resv	= XFS_AG_RESV_NONE,
> +		.prod	= 1
> +	};

Dumb style note: I usually put onstack structs at the top of the
variable declaration list so that the variables are sorted in order of
decreasing size.

> +
> +	ASSERT(id->agno == mp->m_sb.sb_agcount - 1);
> +	error = xfs_ialloc_read_agi(mp, tp, id->agno, &agibp);
> +	if (error)
> +		return error;
> +
> +	agi = agibp->b_addr;
> +
> +	error = xfs_alloc_read_agf(mp, tp, id->agno, 0, &agfbp);
> +	if (error)
> +		return error;
> +
> +	args.fsbno = XFS_AGB_TO_FSB(mp, id->agno,
> +				    be32_to_cpu(agi->agi_length) - len);
> +
> +	/* remove the preallocations before allocation and re-establish then */
> +	error = xfs_ag_resv_free(agibp->b_pag);
> +	if (error)
> +		return error;
> +
> +	/* internal log shouldn't also show up in the free space btrees */
> +	error = xfs_alloc_vextent(&args);
> +	if (error)
> +		goto out;
> +
> +	if (args.agbno == NULLAGBLOCK) {
> +		error = -ENOSPC;
> +		goto out;
> +	}

Hmm.  So if the allocation above takes the last free space in the AG, we
won't be able to satisfy the new per-AG allocation below, which could
lead to a fs shutdown later (and even after subsequent mount attempts)
until enough space gets freed.  That's not good.

I think we should update the length fields in agibp and agfbp, and then
try to re-initialize the per-ag reservation.  If that succeeds, we can
log the agi and agf and return.  If the reinitialization returns a
non-ENOSPC error code then we of course just return the error code and
let the fs shut down.

However, if the reinit returns ENOSPC, then we have to back out
everything we've changed so far.  That means restore the old values of
the agibp/agfbp lengths, log an EFI to free the space we just allocated,
and return.  The caller then has to be smart enough to commit the
transaction before passing the ENOSPC back to its own caller.

> +
> +	/* Change the agi length */
> +	be32_add_cpu(&agi->agi_length, -len);
> +	xfs_ialloc_log_agi(tp, agibp, XFS_AGI_LENGTH);
> +
> +	/* Change agf length */
> +	agf = agfbp->b_addr;
> +	be32_add_cpu(&agf->agf_length, -len);
> +	ASSERT(agf->agf_length == agi->agi_length);

Maybe we should check that these two lengths are the same right after we
grab the AG[IF] buffers and return EFSCORRUPTED?

> +	xfs_alloc_log_agf(tp, agfbp, XFS_AGF_LENGTH);
> +
> +out:
> +	err2 = xfs_ag_resv_init(agibp->b_pag, tp);
> +	if (err2 && err2 != -ENOSPC) {

I think we need to warn if err2 is ENOSPC here, since we're now running
with a (slightly) compromised filesystem.

> +		xfs_warn(mp,
> +"Error %d reserving per-AG metadata reserve pool.", err2);
> +		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
> +		return err2;
> +	}
> +	return error;
> +}
> +
>  /*
>   * Extent the AG indicated by the @id by the length passed in
>   */
> diff --git a/fs/xfs/libxfs/xfs_ag.h b/fs/xfs/libxfs/xfs_ag.h
> index 5166322807e7..f3b5bbfeadce 100644
> --- a/fs/xfs/libxfs/xfs_ag.h
> +++ b/fs/xfs/libxfs/xfs_ag.h
> @@ -24,6 +24,8 @@ struct aghdr_init_data {
>  };
>  
>  int xfs_ag_init_headers(struct xfs_mount *mp, struct aghdr_init_data *id);
> +int xfs_ag_shrink_space(struct xfs_mount *mp, struct xfs_trans *tp,
> +			struct aghdr_init_data *id, xfs_extlen_t len);
>  int xfs_ag_extend_space(struct xfs_mount *mp, struct xfs_trans *tp,
>  			struct aghdr_init_data *id, xfs_extlen_t len);
>  int xfs_ag_get_geometry(struct xfs_mount *mp, xfs_agnumber_t agno,
> diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
> index 8fde7a2989ce..6ee9ea4d5a67 100644
> --- a/fs/xfs/xfs_fsops.c
> +++ b/fs/xfs/xfs_fsops.c
> @@ -26,7 +26,7 @@ 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;
> @@ -76,22 +76,28 @@ 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);
> @@ -99,10 +105,12 @@ xfs_growfs_data_private(
>  	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;
>  	}
> +
>  	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);
>  
> -	/* 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) {
> -		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);
> +		}
> +
>  		if (error)
>  			goto out_trans_cancel;
>  	}
> @@ -137,11 +157,19 @@ 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);
>  	if (id.nfree)
>  		xfs_trans_mod_sb(tp, XFS_TRANS_SB_FDBLOCKS, id.nfree);
> +
> +	/*
> +	 * update in-core counters (especially sb_fdblocks) now
> +	 * so xfs_validate_sb_write() can pass.
> +	 */
> +	if (xfs_sb_version_haslazysbcount(&mp->m_sb))
> +		xfs_log_sb(tp);
> +
>  	xfs_trans_set_sync(tp);
>  	error = xfs_trans_commit(tp);
>  	if (error)
> @@ -157,7 +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 (delta > 0) {
>  		struct xfs_perag	*pag;
>  
>  		pag = xfs_perag_get(mp, id.agno);
> @@ -178,6 +206,10 @@ xfs_growfs_data_private(
>  	return error;
>  
>  out_trans_cancel:
> +	if (!extend && (tp->t_flags & XFS_TRANS_DIRTY)) {

If you're going to have a conditional commit in what looks like the
error return cleanup path, it would be a good idea to leave a comment
here explaining when we can end up in this condition.

--D

> +		xfs_trans_commit(tp);
> +		return error;
> +	}
>  	xfs_trans_cancel(tp);
>  	return error;
>  }
> 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
>