xfs: update freeblocks counter after extent deletion

xfs_bunmapi() currently updates the fdblocks counter, unreserves quota,
etc. before the extent is deleted by xfs_bmap_del_extent(). The function
has problems dividing up the indirect reserved blocks for scenarios
where a single delalloc extent is split in two. Particularly, there
aren't always enough blocks reserved for multiple extents in a single
extent reservation.

The solution to this problem is to allow the extent removal code to
steal from the deleted extent to meet indirect reservation requirements.
Move the block of code in xfs_bmapi() that updates the fdblocks counter
to after the call to xfs_bmap_del_extent() to allow the codepath to
update the extent record before the free blocks are accounted. Also,
reshuffle the code slightly so the delalloc accounting occurs near the
xfs_bmap_del_extent() call to provide context for the comments.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_bmap.c

@@ -5296,31 +5296,7 @@ xfs_bunmapi(
 				goto nodelete;
 			}
 		}
-		if (wasdel) {
-			ASSERT(startblockval(del.br_startblock) > 0);
-			/* Update realtime/data freespace, unreserve quota */
-			if (isrt) {
-				xfs_filblks_t rtexts;
 
-				rtexts = XFS_FSB_TO_B(mp, del.br_blockcount);
-				do_div(rtexts, mp->m_sb.sb_rextsize);
-				xfs_mod_frextents(mp, (int64_t)rtexts);
-				(void)xfs_trans_reserve_quota_nblks(NULL,
-					ip, -((long)del.br_blockcount), 0,
-					XFS_QMOPT_RES_RTBLKS);
-			} else {
-				xfs_mod_fdblocks(mp, (int64_t)del.br_blockcount,
-						 false);
-				(void)xfs_trans_reserve_quota_nblks(NULL,
-					ip, -((long)del.br_blockcount), 0,
-					XFS_QMOPT_RES_REGBLKS);
-			}
-			ip->i_delayed_blks -= del.br_blockcount;
-			if (cur)
-				cur->bc_private.b.flags |=
-					XFS_BTCUR_BPRV_WASDEL;
-		} else if (cur)
-			cur->bc_private.b.flags &= ~XFS_BTCUR_BPRV_WASDEL;
 		/*
 		 * If it's the case where the directory code is running
 		 * with no block reservation, and the deleted block is in
@@ -5342,11 +5318,45 @@ xfs_bunmapi(
 			error = -ENOSPC;
 			goto error0;
 		}
+
+		/*
+		 * Unreserve quota and update realtime free space, if
+		 * appropriate. If delayed allocation, update the inode delalloc
+		 * counter now and wait to update the sb counters as
+		 * xfs_bmap_del_extent() might need to borrow some blocks.
+		 */
+		if (wasdel) {
+			ASSERT(startblockval(del.br_startblock) > 0);
+			if (isrt) {
+				xfs_filblks_t rtexts;
+
+				rtexts = XFS_FSB_TO_B(mp, del.br_blockcount);
+				do_div(rtexts, mp->m_sb.sb_rextsize);
+				xfs_mod_frextents(mp, (int64_t)rtexts);
+				(void)xfs_trans_reserve_quota_nblks(NULL,
+					ip, -((long)del.br_blockcount), 0,
+					XFS_QMOPT_RES_RTBLKS);
+			} else {
+				(void)xfs_trans_reserve_quota_nblks(NULL,
+					ip, -((long)del.br_blockcount), 0,
+					XFS_QMOPT_RES_REGBLKS);
+			}
+			ip->i_delayed_blks -= del.br_blockcount;
+			if (cur)
+				cur->bc_private.b.flags |=
+					XFS_BTCUR_BPRV_WASDEL;
+		} else if (cur)
+			cur->bc_private.b.flags &= ~XFS_BTCUR_BPRV_WASDEL;
+
 		error = xfs_bmap_del_extent(ip, tp, &lastx, flist, cur, &del,
 				&tmp_logflags, whichfork);
 		logflags |= tmp_logflags;
 		if (error)
 			goto error0;
+
+		if (!isrt && wasdel)
+			xfs_mod_fdblocks(mp, (int64_t)del.br_blockcount, false);
+
 		bno = del.br_startoff - 1;
 nodelete:
 		/*