Re: [PATCH v4 1/3] xfs: unconditionally read all AGFs on mounts with perag reservation

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

On 23 Apr 2021 at 18:40, Brian Foster wrote:
> perag reservation is enabled at mount time on a per AG basis. The
> upcoming change to set aside allocbt blocks from block reservation
> requires a populated allocbt counter as soon as possible after mount
> to be fully effective against large perag reservations. Therefore as
> a preparation step, initialize the pagf on all mounts where at least
> one reservation is active. Note that this already occurs to some
> degree on most default format filesystems as reservation requirement
> calculations already depend on the AGF or AGI, depending on the
> reservation type.
>

The changes look good to me.

Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/libxfs/xfs_ag_resv.c | 34 +++++++++++++++++++++++-----------
>  1 file changed, 23 insertions(+), 11 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_ag_resv.c b/fs/xfs/libxfs/xfs_ag_resv.c
> index 6c5f8d10589c..e32a1833d523 100644
> --- a/fs/xfs/libxfs/xfs_ag_resv.c
> +++ b/fs/xfs/libxfs/xfs_ag_resv.c
> @@ -253,7 +253,8 @@ xfs_ag_resv_init(
>  	xfs_agnumber_t			agno = pag->pag_agno;
>  	xfs_extlen_t			ask;
>  	xfs_extlen_t			used;
> -	int				error = 0;
> +	int				error = 0, error2;
> +	bool				has_resv = false;
>  
>  	/* Create the metadata reservation. */
>  	if (pag->pag_meta_resv.ar_asked == 0) {
> @@ -291,6 +292,8 @@ xfs_ag_resv_init(
>  			if (error)
>  				goto out;
>  		}
> +		if (ask)
> +			has_resv = true;
>  	}
>  
>  	/* Create the RMAPBT metadata reservation */
> @@ -304,19 +307,28 @@ xfs_ag_resv_init(
>  		error = __xfs_ag_resv_init(pag, XFS_AG_RESV_RMAPBT, ask, used);
>  		if (error)
>  			goto out;
> +		if (ask)
> +			has_resv = true;
>  	}
>  
> -#ifdef DEBUG
> -	/* need to read in the AGF for the ASSERT below to work */
> -	error = xfs_alloc_pagf_init(pag->pag_mount, tp, pag->pag_agno, 0);
> -	if (error)
> -		return error;
> -
> -	ASSERT(xfs_perag_resv(pag, XFS_AG_RESV_METADATA)->ar_reserved +
> -	       xfs_perag_resv(pag, XFS_AG_RESV_RMAPBT)->ar_reserved <=
> -	       pag->pagf_freeblks + pag->pagf_flcount);
> -#endif
>  out:
> +	/*
> +	 * Initialize the pagf if we have at least one active reservation on the
> +	 * AG. This may have occurred already via reservation calculation, but
> +	 * fall back to an explicit init to ensure the in-core allocbt usage
> +	 * counters are initialized as soon as possible. This is important
> +	 * because filesystems with large perag reservations are susceptible to
> +	 * free space reservation problems that the allocbt counter is used to
> +	 * address.
> +	 */
> +	if (has_resv) {
> +		error2 = xfs_alloc_pagf_init(mp, tp, pag->pag_agno, 0);
> +		if (error2)
> +			return error2;
> +		ASSERT(xfs_perag_resv(pag, XFS_AG_RESV_METADATA)->ar_reserved +
> +		       xfs_perag_resv(pag, XFS_AG_RESV_RMAPBT)->ar_reserved <=
> +		       pag->pagf_freeblks + pag->pagf_flcount);
> +	}
>  	return error;
>  }

-- 
chandan