Re: [PATCH] xfs: perag initialization should only touch m_ag_max_usable for AG 0

[Brian Foster <bfoster@redhat.com>]

On Tue, Sep 19, 2017 at 11:10:41AM -0700, Darrick J. Wong wrote:
> We call __xfs_ag_resv_init to make a per-AG reservation for each AG.
> This makes the reservation per-AG, not per-filesystem.  Therefore, it
> is incorrect to adjust m_ag_max_usable for each AG.  Adjust it only
> when we're reserving AG 0's blocks so that we only do it once per fs.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/xfs/libxfs/xfs_ag_resv.c |   12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_ag_resv.c b/fs/xfs/libxfs/xfs_ag_resv.c
> index b008ff3..df3e600 100644
> --- a/fs/xfs/libxfs/xfs_ag_resv.c
> +++ b/fs/xfs/libxfs/xfs_ag_resv.c
> @@ -156,7 +156,8 @@ __xfs_ag_resv_free(
>  	trace_xfs_ag_resv_free(pag, type, 0);
>  
>  	resv = xfs_perag_resv(pag, type);
> -	pag->pag_mount->m_ag_max_usable += resv->ar_asked;
> +	if (pag->pag_agno == 0)
> +		pag->pag_mount->m_ag_max_usable += resv->ar_asked;
>  	/*
>  	 * AGFL blocks are always considered "free", so whatever
>  	 * was reserved at mount time must be given back at umount.
> @@ -216,7 +217,14 @@ __xfs_ag_resv_init(
>  		return error;
>  	}
>  
> -	mp->m_ag_max_usable -= ask;
> +	/*
> +	 * Reduce the maximum per-AG allocation length by however much we're
> +	 * trying to reserve for an AG.  Since this is a filesystem-wide
> +	 * counter, we only make the adjustment for AG 0.  This assumes that
> +	 * there aren't any AGs hungrier for per-AG reservation than AG 0.
> +	 */
> +	if (pag->pag_agno == 0)
> +		mp->m_ag_max_usable -= ask;
>  
>  	resv = xfs_perag_resv(pag, type);
>  	resv->ar_asked = ask;
> --
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