xfs: fix reflink quota reservation accounting error

Quota reservations are supposed to account for the blocks that might be
allocated due to a bmap btree split.  Reflink doesn't do this, so fix
this to make the quota accounting more accurate before we start
rearranging things.

Fixes: 862bb360 ("xfs: reflink extents from one file to another")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/xfs_reflink.c

@@ -1003,6 +1003,7 @@ xfs_reflink_remap_extent(
 	xfs_filblks_t		rlen;
 	xfs_filblks_t		unmap_len;
 	xfs_off_t		newlen;
+	int64_t			qres;
 	int			error;
 
 	unmap_len = irec->br_startoff + irec->br_blockcount - destoff;
@@ -1025,13 +1026,19 @@ xfs_reflink_remap_extent(
 	xfs_ilock(ip, XFS_ILOCK_EXCL);
 	xfs_trans_ijoin(tp, ip, 0);
 
-	/* If we're not just clearing space, then do we have enough quota? */
-	if (real_extent) {
-		error = xfs_trans_reserve_quota_nblks(tp, ip,
-				irec->br_blockcount, 0, XFS_QMOPT_RES_REGBLKS);
+	/*
+	 * Reserve quota for this operation.  We don't know if the first unmap
+	 * in the dest file will cause a bmap btree split, so we always reserve
+	 * at least enough blocks for that split.  If the extent being mapped
+	 * in is written, we need to reserve quota for that too.
+	 */
+	qres = XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK);
+	if (real_extent)
+		qres += irec->br_blockcount;
+	error = xfs_trans_reserve_quota_nblks(tp, ip, qres, 0,
+			XFS_QMOPT_RES_REGBLKS);
 	if (error)
 		goto out_cancel;
-	}
 
 	trace_xfs_reflink_remap(ip, irec->br_startoff,
 				irec->br_blockcount, irec->br_startblock);