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

[Chandan Babu R <chandanrlinux@gmail.com>]

On 23 Apr 2021 at 18:40, 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>
> ---
>  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)) {

pag->pagf_btreeblks gets incremented everytime a block is allocated to refill
AGFL (via xfs_alloc_get_freelist()). Apart from the allobt trees, blocks for
Rmap btree also get allocated from AGFL. Hence pag->pagf_btreeblks must be
larger than agf->agf_rmap_blocks.

Can you please describe the scenario in which pag->pagf_btreeblks has a value
that is <= agf->agf_rmap_blocks?

--
chandan