xfs: support shrinking unused space in the last AG

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.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_fsops.c

@@ -91,50 +91,74 @@ 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;
 	bool			lastag_extended;
 	xfs_agnumber_t		oagcount;
 	struct xfs_trans	*tp;
 	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);
+	}
 
 	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)
-			return -EINVAL;
 	}
 	delta = nb - mp->m_sb.sb_dblocks;
+	/*
+	 * Reject filesystems with a single AG because they are not
+	 * supported, and reject a shrink operation that would cause a
+	 * filesystem to become unsupported.
+	 */
+	if (delta < 0 && nagcount < 2)
+		return -EINVAL;
+
 	oagcount = mp->m_sb.sb_agcount;
 
 	/* allocate the new per-ag structures */
 	if (nagcount > oagcount) {
 		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);
+			(delta > 0 ? XFS_GROWFS_SPACE_RES(mp) : -delta), 0,
+			XFS_TRANS_RESERVE, &tp);
 	if (error)
 		return error;
 
-	error = xfs_resizefs_init_new_ags(tp, &id, oagcount, nagcount,
-					  delta, &lastag_extended);
+	if (delta > 0) {
+		error = xfs_resizefs_init_new_ags(tp, &id, oagcount, nagcount,
+						  delta, &lastag_extended);
+	} else {
+		static struct ratelimit_state shrink_warning = \
+			RATELIMIT_STATE_INIT("shrink_warning", 86400 * HZ, 1);
+		ratelimit_set_flags(&shrink_warning, RATELIMIT_MSG_ON_RELEASE);
+
+		if (__ratelimit(&shrink_warning))
+			xfs_alert(mp,
+	"EXPERIMENTAL online shrink feature in use. Use at your own risk!");
+
+		error = xfs_ag_shrink_space(mp, &tp, nagcount - 1, -delta);
+	}
 	if (error)
 		goto out_trans_cancel;
 
@@ -169,28 +193,29 @@ xfs_growfs_data_private(
 	xfs_set_low_space_thresholds(mp);
 	mp->m_alloc_set_aside = xfs_alloc_set_aside(mp);
 
-	/*
-	 * If we expanded the last AG, free the per-AG reservation
-	 * so we can reinitialize it with the new size.
-	 */
-	if (lastag_extended) {
-		struct xfs_perag	*pag;
-
-		pag = xfs_perag_get(mp, id.agno);
-		error = xfs_ag_resv_free(pag);
-		xfs_perag_put(pag);
-		if (error)
-			return error;
+	if (delta > 0) {
+		/*
+		 * If we expanded the last AG, free the per-AG reservation
+		 * so we can reinitialize it with the new size.
+		 */
+		if (lastag_extended) {
+			struct xfs_perag	*pag;
+
+			pag = xfs_perag_get(mp, id.agno);
+			error = xfs_ag_resv_free(pag);
+			xfs_perag_put(pag);
+			if (error)
+				return error;
+		}
+		/*
+		 * Reserve AG metadata blocks. ENOSPC here does not mean there
+		 * was a growfs failure, just that there still isn't space for
+		 * new user data after the grow has been run.
+		 */
+		error = xfs_fs_reserve_ag_blocks(mp);
+		if (error == -ENOSPC)
+			error = 0;
 	}
-
-	/*
-	 * Reserve AG metadata blocks. ENOSPC here does not mean there was a
-	 * growfs failure, just that there still isn't space for new user data
-	 * after the grow has been run.
-	 */
-	error = xfs_fs_reserve_ag_blocks(mp);
-	if (error == -ENOSPC)
-		error = 0;
 	return error;
 
 out_trans_cancel:

fs/xfs/xfs_trans.c

@@ -436,7 +436,6 @@ 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: