Re: [PATCH v5 08/10] xfs: Update atomic write max size

[Carlos Maiolino]

Thanks for updating it John.

Reviewed-by: Carlos Maiolino <cmaiolino@xxxxxxxxxx>

On Mon, Mar 10, 2025 at 06:39:44PM +0000, John Garry wrote:
> Now that CoW-based atomic writes are supported, update the max size of an
> atomic write.
> 
> For simplicity, limit at the max of what the mounted bdev can support in
> terms of atomic write limits. Maybe in future we will have a better way
> to advertise this optimised limit.
> 
> In addition, the max atomic write size needs to be aligned to the agsize.
> Limit the size of atomic writes to the greatest power-of-two factor of the
> agsize so that allocations for an atomic write will always be aligned
> compatibly with the alignment requirements of the storage.
> 
> For RT inode, just limit to 1x block, even though larger can be supported
> in future.
> 
> Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_iops.c  | 14 +++++++++++++-
>  fs/xfs/xfs_mount.c | 28 ++++++++++++++++++++++++++++
>  fs/xfs/xfs_mount.h |  1 +
>  3 files changed, 42 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index de065cc2e7cf..16a1f9541690 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -607,12 +607,24 @@ xfs_get_atomic_write_attr(
>  	unsigned int		*unit_min,
>  	unsigned int		*unit_max)
>  {
> +	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> +	struct xfs_mount	*mp = ip->i_mount;
> +
>  	if (!xfs_inode_can_atomicwrite(ip)) {
>  		*unit_min = *unit_max = 0;
>  		return;
>  	}
> 
> -	*unit_min = *unit_max = ip->i_mount->m_sb.sb_blocksize;
> +	*unit_min = ip->i_mount->m_sb.sb_blocksize;
> +
> +	if (XFS_IS_REALTIME_INODE(ip)) {
> +		/* For now, set limit at 1x block */
> +		*unit_max = ip->i_mount->m_sb.sb_blocksize;
> +	} else {
> +		*unit_max =  min_t(unsigned int,
> +					XFS_FSB_TO_B(mp, mp->m_awu_max),
> +					target->bt_bdev_awu_max);
> +	}
>  }
> 
>  static void
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index e65a659901d5..414adfb944b9 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -665,6 +665,32 @@ xfs_agbtree_compute_maxlevels(
>  	levels = max(levels, mp->m_rmap_maxlevels);
>  	mp->m_agbtree_maxlevels = max(levels, mp->m_refc_maxlevels);
>  }
> +static inline void
> +xfs_compute_awu_max(
> +	struct xfs_mount	*mp)
> +{
> +	xfs_agblock_t		agsize = mp->m_sb.sb_agblocks;
> +	xfs_agblock_t		awu_max;
> +
> +	if (!xfs_has_reflink(mp)) {
> +		mp->m_awu_max = 1;
> +		return;
> +	}
> +
> +	/*
> +	 * Find highest power-of-2 evenly divisible into agsize and which
> +	 * also fits into an unsigned int field.
> +	 */
> +	awu_max = 1;
> +	while (1) {
> +		if (agsize % (awu_max * 2))
> +			break;
> +		if (XFS_FSB_TO_B(mp, awu_max * 2) > UINT_MAX)
> +			break;
> +		awu_max *= 2;
> +	}
> +	mp->m_awu_max = awu_max;
> +}
> 
>  /* Compute maximum possible height for realtime btree types for this fs. */
>  static inline void
> @@ -751,6 +777,8 @@ xfs_mountfs(
>  	xfs_agbtree_compute_maxlevels(mp);
>  	xfs_rtbtree_compute_maxlevels(mp);
> 
> +	xfs_compute_awu_max(mp);
> +
>  	/*
>  	 * Check if sb_agblocks is aligned at stripe boundary.  If sb_agblocks
>  	 * is NOT aligned turn off m_dalign since allocator alignment is within
> diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> index 799b84220ebb..1b0136da2aec 100644
> --- a/fs/xfs/xfs_mount.h
> +++ b/fs/xfs/xfs_mount.h
> @@ -229,6 +229,7 @@ typedef struct xfs_mount {
>  	bool			m_finobt_nores; /* no per-AG finobt resv. */
>  	bool			m_update_sb;	/* sb needs update in mount */
>  	unsigned int		m_max_open_zones;
> +	xfs_extlen_t		m_awu_max;	/* data device max atomic write */
> 
>  	/*
>  	 * Bitsets of per-fs metadata that have been checked and/or are sick.
> --
> 2.31.1
>