Re: [PATCH 1/2] xfs: validate that zoned RT devices are zone aligned

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Dec 15, 2025 at 10:48:36AM +0100, Christoph Hellwig wrote:
> Garbage collection assumes all zones contain the full amount of blocks.
> Mkfs already ensures this happens, but make the kernel check it as well
> to avoid getting into trouble due to fuzzers or mkfs bugs.
> 

If this gets merged, please add this so the new requirements are
autobackported to stable:
Cc: <stable@vger.kernel.org> # v6.15

> Fixes: 2167eaabe2fa ("xfs: define the zoned on-disk format")
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/libxfs/xfs_sb.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> index cdd16dd805d7..db5231f846ea 100644
> --- a/fs/xfs/libxfs/xfs_sb.c
> +++ b/fs/xfs/libxfs/xfs_sb.c
> @@ -301,6 +301,19 @@ xfs_validate_rt_geometry(
>  	    sbp->sb_rbmblocks != xfs_expected_rbmblocks(sbp))
>  		return false;
>  
> +	if (xfs_sb_is_v5(sbp) &&
> +	    (sbp->sb_features_incompat & XFS_SB_FEAT_INCOMPAT_ZONED)) {
> +		uint32_t		mod;
> +
> +		/*
> +		 * Zoned RT devices must be aligned to the rtgroup size, because
> +		 * garbage collection can't deal with rump RT groups.

I've decided that I'm ok with imposing this new restriction after the
fact, but only because actual zoned hardware will never expose a runt
group, so the only way you could end up with one now is if you formatted
with zoned=1 without a hardware-zoned storage device.

Could this comment be expanded to say that explicitly?

		/*
		 * Zoned RT devices must be aligned to the rtgroup size,
		 * because garbage collection can't deal with rump RT
		 * groups.  Hardware-zoned storage will never expose a
		 * runt group, so this is only possible with soft-zoned
		 * filesystems, which are not created by default.
		 */

With that change (and a corresponding mkfs enforcement patch),
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D


> +		 */
> +		div_u64_rem(sbp->sb_rextents, sbp->sb_rgextents, &mod);
> +		if (mod)
> +			return false;
> +	}
> +
>  	return true;
>  }
>  
> -- 
> 2.47.3
> 
>