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

[Allison Henderson <allison.henderson@oracle.com>]

On 4/23/21 6:10 AM, Brian Foster wrote:
> 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>
OK, makes sense
Reviewed-by: Allison Henderson <allison.henderson@oracle.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 */
>