xfs: unconditionally read all AGFs on mounts with perag reservation

perag reservation is enabled at mount time on a per AG basis. The
upcoming change to set aside allocbt blocks from block reservation
requires a populated allocbt counter as soon as possible after mount
to be fully effective against large perag reservations. Therefore as
a preparation step, initialize the pagf on all mounts where at least
one reservation is active. Note that this already occurs to some
degree on most default format filesystems as reservation requirement
calculations already depend on the AGF or AGI, depending on the
reservation type.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/libxfs/xfs_ag_resv.c

@@ -253,7 +253,8 @@ xfs_ag_resv_init(
 	xfs_agnumber_t			agno = pag->pag_agno;
 	xfs_extlen_t			ask;
 	xfs_extlen_t			used;
-	int				error = 0;
+	int				error = 0, error2;
+	bool				has_resv = false;
 
 	/* Create the metadata reservation. */
 	if (pag->pag_meta_resv.ar_asked == 0) {
@@ -291,6 +292,8 @@ xfs_ag_resv_init(
 			if (error)
 				goto out;
 		}
+		if (ask)
+			has_resv = true;
 	}
 
 	/* Create the RMAPBT metadata reservation */
@@ -304,19 +307,28 @@ xfs_ag_resv_init(
 		error = __xfs_ag_resv_init(pag, XFS_AG_RESV_RMAPBT, ask, used);
 		if (error)
 			goto out;
+		if (ask)
+			has_resv = true;
 	}
 
-#ifdef DEBUG
-	/* need to read in the AGF for the ASSERT below to work */
-	error = xfs_alloc_pagf_init(pag->pag_mount, tp, pag->pag_agno, 0);
-	if (error)
-		return error;
-
-	ASSERT(xfs_perag_resv(pag, XFS_AG_RESV_METADATA)->ar_reserved +
-	       xfs_perag_resv(pag, XFS_AG_RESV_RMAPBT)->ar_reserved <=
-	       pag->pagf_freeblks + pag->pagf_flcount);
-#endif
 out:
+	/*
+	 * Initialize the pagf if we have at least one active reservation on the
+	 * AG. This may have occurred already via reservation calculation, but
+	 * fall back to an explicit init to ensure the in-core allocbt usage
+	 * counters are initialized as soon as possible. This is important
+	 * because filesystems with large perag reservations are susceptible to
+	 * free space reservation problems that the allocbt counter is used to
+	 * address.
+	 */
+	if (has_resv) {
+		error2 = xfs_alloc_pagf_init(mp, tp, pag->pag_agno, 0);
+		if (error2)
+			return error2;
+		ASSERT(xfs_perag_resv(pag, XFS_AG_RESV_METADATA)->ar_reserved +
+		       xfs_perag_resv(pag, XFS_AG_RESV_RMAPBT)->ar_reserved <=
+		       pag->pagf_freeblks + pag->pagf_flcount);
+	}
 	return error;
 }