xfs: fix log intent recovery ENOSPC shutdowns when inactivating inodes

During regular operation, the xfs_inactive operations create
transactions with zero block reservation because in general we're
freeing space, not asking for more.  The per-AG space reservations
created at mount time enable us to handle expansions of the refcount
btree without needing to reserve blocks to the transaction.

Unfortunately, log recovery doesn't create the per-AG space reservations
when intent items are being recovered.  This isn't an issue for intent
item recovery itself because they explicitly request blocks, but any
inode inactivation that can happen during log recovery uses the same
xfs_inactive paths as regular runtime.  If a refcount btree expansion
happens, the transaction will fail due to blk_res_used > blk_res, and we
shut down the filesystem unnecessarily.

Fix this problem by making per-AG reservations temporarily so that we
can handle the inactivations, and releasing them at the end.  This
brings the recovery environment closer to the runtime environment.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_mount.c

@@ -859,9 +859,17 @@ xfs_mountfs(
 	/*
 	 * Finish recovering the file system.  This part needed to be delayed
 	 * until after the root and real-time bitmap inodes were consistently
-	 * read in.
+	 * read in.  Temporarily create per-AG space reservations for metadata
+	 * btree shape changes because space freeing transactions (for inode
+	 * inactivation) require the per-AG reservation in lieu of reserving
+	 * blocks.
 	 */
+	error = xfs_fs_reserve_ag_blocks(mp);
+	if (error && error == -ENOSPC)
+		xfs_warn(mp,
+	"ENOSPC reserving per-AG metadata pool, log recovery may fail.");
 	error = xfs_log_mount_finish(mp);
+	xfs_fs_unreserve_ag_blocks(mp);
 	if (error) {
 		xfs_warn(mp, "log mount finish failed");
 		goto out_rtunmount;