Re: [PATCH v2 1/2] nilfs2: fix 64-bit division operations in nilfs_bmap_find_target_in_group()

[David Laight <david.laight.linux@gmail.com>]

On Fri, 13 Mar 2026 14:45:20 -0400
Jeff Layton <jlayton@kernel.org> wrote:

> With the change to make inode->i_ino a u64, the build started failing on
> 32-bit ARM with:
> 
>     ERROR: modpost: "__aeabi_uldivmod" [fs/nilfs2/nilfs2.ko] undefined!
> 
> Fix this by using the 64-bit division interfaces in
> nilfs_bmap_find_target_in_group().
> 
> Fixes: 998a59d371c2 ("treewide: fix missed i_ino format specifier conversions")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202603100602.KPxiClIO-lkp@intel.com/
> Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
> Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/nilfs2/bmap.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/nilfs2/bmap.c b/fs/nilfs2/bmap.c
> index 824f2bd91c167965ec3a660202b6e6c5f1fe007e..abcf5252578ad24f694bfccf525893674bfcb4bc 100644
> --- a/fs/nilfs2/bmap.c
> +++ b/fs/nilfs2/bmap.c
> @@ -455,11 +455,14 @@ __u64 nilfs_bmap_find_target_in_group(const struct nilfs_bmap *bmap)
>  {
>  	struct inode *dat = nilfs_bmap_get_dat(bmap);
>  	unsigned long entries_per_group = nilfs_palloc_entries_per_group(dat);
> -	unsigned long group = bmap->b_inode->i_ino / entries_per_group;

Are you sure entries_per_group can be more than 32 bits?
It looks like something that will be the same size on 32 and 64bit.

> +	unsigned long group;
> +	u32 index;
> +
> +	group = div_u64(bmap->b_inode->i_ino, entries_per_group);

You don't need the full 64 by 64 divide.
IIRC there are both div_u64_u32() and div_u64_ulong().

> +	div_u64_rem(bmap->b_inode->i_ino, NILFS_BMAP_GROUP_DIV, &index);

NILFD_BMAP_GROUP_DIV is 8 (and probably has to be a power of 2).
So:
	index = bmap->b_inode->i_ino & (NILFS_BMAP_GROUP_DIV - 1);
is the same and likely much faster to calculate.
(The compiler will have done that optimisation before.)

	David


>  
>  	return group * entries_per_group +
> -		(bmap->b_inode->i_ino % NILFS_BMAP_GROUP_DIV) *
> -		(entries_per_group / NILFS_BMAP_GROUP_DIV);
> +	       index * (entries_per_group / NILFS_BMAP_GROUP_DIV);
>  }
>  
>  static struct lock_class_key nilfs_bmap_dat_lock_key;
>