Re: [PATCH] XFS: fix zoned gc threshold math for 32-bit arches

[Hans Holmberg <Hans.Holmberg@wdc.com>]

On 22/04/2025 13:42, cem@kernel.org wrote:
> From: Carlos Maiolino <cem@kernel.org>
> 
> xfs_zoned_need_gc makes use of mult_frac() to calculate the threshold
> for triggering the zoned garbage collector, but, turns out mult_frac()
> doesn't properly work with 64-bit data types and this caused build
> failures on some 32-bit architectures.
> 
> Fix this by essentially open coding mult_frac() in a 64-bit friendly
> way.
> 
> Notice we don't need to bother with counters underflow here because
> xfs_estimate_freecounter() will always return a positive value, as it
> leverages percpu_counter_read_positive to read such counters.
> 
> Fixes: 845abeb1f06a ("xfs: add tunable threshold parameter for triggering zone GC")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202504181233.F7D9Atra-lkp@intel.com/
> Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
> ---
> 
>  fs/xfs/xfs_zone_gc.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_zone_gc.c b/fs/xfs/xfs_zone_gc.c
> index 8c541ca71872..b0e8915ef733 100644
> --- a/fs/xfs/xfs_zone_gc.c
> +++ b/fs/xfs/xfs_zone_gc.c
> @@ -171,6 +171,7 @@ xfs_zoned_need_gc(
>  	struct xfs_mount	*mp)
>  {
>  	s64			available, free;
> +	s32			threshold, remainder;
>  
>  	if (!xfs_group_marked(mp, XG_TYPE_RTG, XFS_RTG_RECLAIMABLE))
>  		return false;
> @@ -183,7 +184,12 @@ xfs_zoned_need_gc(
>  		return true;
>  
>  	free = xfs_estimate_freecounter(mp, XC_FREE_RTEXTENTS);
> -	if (available < mult_frac(free, mp->m_zonegc_low_space, 100))
> +
> +	threshold = div_s64_rem(free, 100, &remainder);

Hmm, shouldn't threshold be a s64?

> +	threshold = threshold * mp->m_zonegc_low_space +
> +		    remainder * div_s64(mp->m_zonegc_low_space, 100);
> +
> +	if (available < threshold)
>  		return true;
>  
>  	return false;