[PATCH v4 2/3] xfs: introduce in-core global counter of allocbt blocks

[Brian Foster <bfoster@redhat.com>]

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>
---
 fs/xfs/libxfs/xfs_alloc.c       | 12 ++++++++++++
 fs/xfs/libxfs/xfs_alloc_btree.c |  2 ++
 fs/xfs/xfs_mount.h              |  6 ++++++
 3 files changed, 20 insertions(+)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index aaa19101bb2a..144e2d68245c 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -3036,6 +3036,7 @@ xfs_alloc_read_agf(
 	struct xfs_agf		*agf;		/* ag freelist header */
 	struct xfs_perag	*pag;		/* per allocation group data */
 	int			error;
+	uint32_t		allocbt_blks;
 
 	trace_xfs_alloc_read_agf(mp, agno);
 
@@ -3066,6 +3067,17 @@ 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 global in-core allocbt block counter. Filter
+		 * rmapbt blocks from the on-disk counter because those are
+		 * managed by perag reservation.
+		 */
+		if (pag->pagf_btreeblks > be32_to_cpu(agf->agf_rmap_blocks)) {
+			allocbt_blks = pag->pagf_btreeblks -
+					be32_to_cpu(agf->agf_rmap_blocks);
+			atomic64_add(allocbt_blks, &mp->m_allocbt_blks);
+		}
 	}
 #ifdef DEBUG
 	else if (!XFS_FORCED_SHUTDOWN(mp)) {
diff --git a/fs/xfs/libxfs/xfs_alloc_btree.c b/fs/xfs/libxfs/xfs_alloc_btree.c
index 8e01231b308e..9f5a45f7baed 100644
--- a/fs/xfs/libxfs/xfs_alloc_btree.c
+++ b/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);
 
 	xfs_trans_agbtree_delta(cur->bc_tp, 1);
@@ -95,6 +96,7 @@ 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);
 	xfs_trans_agbtree_delta(cur->bc_tp, -1);
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index 81829d19596e..bb67274ee23f 100644
--- a/fs/xfs/xfs_mount.h
+++ b/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 */
-- 
2.26.3