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

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

On Wed, Dec 10, 2025 at 03:23:05PM +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

mkfs doesn't enforce that when you're creating a zoned filesystem on
non-zoned storage:

# mkfs.xfs -r rtdev=/dev/sda,zoned=1 -f /dev/sdf
meta-data=/dev/sdf               isize=512    agcount=4, agsize=1298176 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=1, sparse=1, rmapbt=1
         =                       reflink=0    bigtime=1 inobtcount=1 nrext64=1
         =                       exchange=1   metadir=1
data     =                       bsize=4096   blocks=5192704, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1, parent=1
log      =internal log           bsize=4096   blocks=16384, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =/dev/sda               extsz=4096   blocks=5192704, rtextents=5192704
         =                       rgcount=80   rgsize=65536 extents
         =                       zoned=1      start=0 reserved=0

5192704 isn't congruent with 65536, and we get a runt rtgroup at the
end:

# mount /dev/sdf /opt -o rtdev=/dev/sda
# xfs_io -c 'rginfo' /opt | tail -n 20
RTG: 76
Length: 65536
Sick: 0x0
Checked: 0x0
Flags: 0x0
RTG: 77
Length: 65536
Sick: 0x0
Checked: 0x0
Flags: 0x0
RTG: 78
Length: 65536
Sick: 0x0
Checked: 0x0
Flags: 0x0
RTG: 79
Length: 15360
Sick: 0x0
Checked: 0x0
Flags: 0x0

rtgroup 79 is clearly a runt group.

(The mkfs enforcement does work if you have an actual zoned storage
device since mkfs complains about changes in the zone sizes.)

> to avoid getting into trouble due to fuzzers or mkfs bugs.
>
> Fixes: 2167eaabe2fa ("xfs: define the zoned on-disk format")
> Signed-off-by: Christoph Hellwig <hch@lst.de>

How many filesystems are there in the wild with rump rtgroups?  My first
thought was "why not pretend the runt rtgroup doesn't exist?" but then
that creates all sorts of weirdness where you have a 778M rt volume on a
disk with 256M rtgroups, but then we ignore the 10M of space and you can
never get to it.

Given that runt zoned rtgroups can exist in the wild, how hard would it
be to fix zonegc?

--D

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