Re: [PATCH 3/3] xfs: set s_min_writeback_pages for zoned file systems

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

On Wed, Oct 15, 2025 at 03:27:16PM +0900, Christoph Hellwig wrote:
> Set s_min_writeback_pages to the zone size, so that writeback always
> writes up to a full zone.  This ensures that writeback does not add
> spurious file fragmentation when writing back a large number of
> files that are larger than the zone size.
> 
> Fixes: 4e4d52075577 ("xfs: add the zoned space allocator")
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_zone_alloc.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_zone_alloc.c b/fs/xfs/xfs_zone_alloc.c
> index 1147bacb2da8..0f4e460fd3ea 100644
> --- a/fs/xfs/xfs_zone_alloc.c
> +++ b/fs/xfs/xfs_zone_alloc.c
> @@ -1215,6 +1215,7 @@ xfs_mount_zones(
>  		.mp		= mp,
>  	};
>  	struct xfs_buftarg	*bt = mp->m_rtdev_targp;
> +	xfs_extlen_t		zone_blocks = mp->m_groups[XG_TYPE_RTG].blocks;
>  	int			error;
>  
>  	if (!bt) {
> @@ -1245,10 +1246,12 @@ xfs_mount_zones(
>  		return -ENOMEM;
>  
>  	xfs_info(mp, "%u zones of %u blocks (%u max open zones)",
> -		 mp->m_sb.sb_rgcount, mp->m_groups[XG_TYPE_RTG].blocks,
> -		 mp->m_max_open_zones);
> +		 mp->m_sb.sb_rgcount, zone_blocks, mp->m_max_open_zones);
>  	trace_xfs_zones_mount(mp);
>  
> +	mp->m_super->s_min_writeback_pages =
> +		XFS_FSB_TO_B(mp, zone_blocks) >> PAGE_SHIFT;

Hmm.  The maximum rtgroup (and hence zone) size is 2^31-1 blocks.
That quantity is casted to int64_t by FSB_TO_B, then shifted down by
PAGE_SHIFT.  So I think there's no chance of an overflow here,
especially if s_min_writeback_pages becomes type long.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> +
>  	if (bdev_is_zoned(bt->bt_bdev)) {
>  		error = blkdev_report_zones(bt->bt_bdev,
>  				XFS_FSB_TO_BB(mp, mp->m_sb.sb_rtstart),
> -- 
> 2.47.3
> 
>