xfs: introduce in-core global counter of allocbt blocks

Introduce an in-core counter to track the sum of all allocbt blocks
used by the filesystem. This value is currently tracked per-ag via
the ->agf_btreeblks field in the AGF, which also happens to include
rmapbt blocks. A global, in-core count of allocbt blocks is required
to identify the subset of global ->m_fdblocks that consists of
unavailable blocks currently used for allocation btrees. To support
this calculation at block reservation time, construct a similar
global counter for allocbt blocks, populate it on first read of each
AGF and update it as allocbt blocks are used and released.

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_alloc.c

@@ -3033,6 +3033,7 @@ xfs_alloc_read_agf(
 	struct xfs_agf		*agf;		/* ag freelist header */
 	struct xfs_perag	*pag;		/* per allocation group data */
 	int			error;
+	int			allocbt_blks;
 
 	trace_xfs_alloc_read_agf(mp, agno);
 
@@ -3063,6 +3064,19 @@ xfs_alloc_read_agf(
 		pag->pagf_refcount_level = be32_to_cpu(agf->agf_refcount_level);
 		pag->pagf_init = 1;
 		pag->pagf_agflreset = xfs_agfl_needs_reset(mp, agf);
+
+		/*
+		 * Update the in-core allocbt counter. Filter out the rmapbt
+		 * subset of the btreeblks counter because the rmapbt is managed
+		 * by perag reservation. Subtract one for the rmapbt root block
+		 * because the rmap counter includes it while the btreeblks
+		 * counter only tracks non-root blocks.
+		 */
+		allocbt_blks = pag->pagf_btreeblks;
+		if (xfs_sb_version_hasrmapbt(&mp->m_sb))
+			allocbt_blks -= be32_to_cpu(agf->agf_rmap_blocks) - 1;
+		if (allocbt_blks > 0)
+			atomic64_add(allocbt_blks, &mp->m_allocbt_blks);
 	}
 #ifdef DEBUG
 	else if (!XFS_FORCED_SHUTDOWN(mp)) {

fs/xfs/libxfs/xfs_alloc_btree.c

@@ -71,6 +71,7 @@ xfs_allocbt_alloc_block(
 		return 0;
 	}
 
+	atomic64_inc(&cur->bc_mp->m_allocbt_blks);
 	xfs_extent_busy_reuse(cur->bc_mp, cur->bc_ag.agno, bno, 1, false);
 
 	new->s = cpu_to_be32(bno);
@@ -94,6 +95,7 @@ xfs_allocbt_free_block(
 	if (error)
 		return error;
 
+	atomic64_dec(&cur->bc_mp->m_allocbt_blks);
 	xfs_extent_busy_insert(cur->bc_tp, be32_to_cpu(agf->agf_seqno), bno, 1,
 			      XFS_EXTENT_BUSY_SKIP_DISCARD);
 	return 0;

fs/xfs/xfs_mount.h

@@ -170,6 +170,12 @@ typedef struct xfs_mount {
 	 * extents or anything related to the rt device.
 	 */
 	struct percpu_counter	m_delalloc_blks;
+	/*
+	 * Global count of allocation btree blocks in use across all AGs. Only
+	 * used when perag reservation is enabled. Helps prevent block
+	 * reservation from attempting to reserve allocation btree blocks.
+	 */
+	atomic64_t		m_allocbt_blks;
 
 	struct radix_tree_root	m_perag_tree;	/* per-ag accounting info */
 	spinlock_t		m_perag_lock;	/* lock for m_perag_tree */