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 <redacted>
---
 fs/xfs/libxfs/xfs_ag.c | 72 ++++++++++++++++++++++++++++++++++++++++++
 fs/xfs/libxfs/xfs_ag.h |  2 ++
 fs/xfs/xfs_fsops.c     | 69 ++++++++++++++++++++++++++++++----------
 fs/xfs/xfs_trans.c     |  1 -
 4 files changed, 126 insertions(+), 18 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
+	};
+
+	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;
+	}
+
+	/* 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);
+	xfs_alloc_log_agf(tp, agfbp, XFS_AGF_LENGTH);
+
+out:
+	err2 = xfs_ag_resv_init(agibp->b_pag, tp);
+	if (err2 && err2 != -ENOSPC) {
+		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 a792d1f0ac55..eea38395e804 100644
--- a/fs/xfs/xfs_fsops.c
+++ b/fs/xfs/xfs_fsops.c

@@ -79,19 +79,22 @@ xfs_growfs_data_private(
 	xfs_rfsblock_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)))
+	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);
+	}
 
 	delta = nb;	/* use delta as a temporary here */

 	nb_mod = do_div(delta, mp->m_sb.sb_agblocks);

@@ -99,10 +102,18 @@ 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;
+
+	if (nb > mp->m_sb.sb_dblocks) {
+		delta = nb - mp->m_sb.sb_dblocks;
+		extend = true;
+	} else {
+		delta = mp->m_sb.sb_dblocks - nb;
+		extend = false;

+	}
+
 	oagcount = mp->m_sb.sb_agcount;
 
 	/* allocate the new per-ag structures */

@@ -110,22 +121,34 @@ 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 +160,19 @@ 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)

@@ -178,6 +209,10 @@ xfs_growfs_data_private(
 	return error;
 
 out_trans_cancel:
+	if (!extend && (tp->t_flags & XFS_TRANS_DIRTY)) {
+		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