Re: [PATCH] xfs: don't limit software atomic writes by the group alignment

[Pankaj Raghav <pankaj.raghav@linux.dev>]

On 9/30/2026 2:48 PM, John Garry wrote:
> On 9/30/26 13:27, Pankaj Raghav wrote:
>>>
>>> Maybe in this case we don't need to restrict CoW-based atomics to 1 fsblock. 
>>> But why care? I mean, you have HW support, which is so much better to use 
>>> than CoW- based atomics, so better to config your FS to avail of them.
>>
>> That is true but it is a bit strange we expose a smaller CoW based atomics 
>> when we have HW based atomic support.
>>
>> For me the main issue is this function:
>> static xfs_extlen_t
>> xfs_calc_group_awu_max(
>>      struct xfs_mount    *mp,
>>      enum xfs_group_type    type)
>> {
>>      struct xfs_groups    *g = &mp->m_groups[type];
>>      struct xfs_buftarg    *btp = xfs_group_type_buftarg(mp, type);
>>
>>      if (g->blocks == 0)
>>          return 0;
>>      if (btp && btp->bt_awu_min > 0)
>>          return max_pow_of_two_factor(g->blocks);
>>      return rounddown_pow_of_two(g->blocks);
>> }
> 
> 
> ----->8-----
> 
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -694,7 +694,7 @@ xfs_calc_group_awu_max(
> 
>          if (g->blocks == 0)
>                  return 0;
> -       if (btp && btp->bt_awu_min > 0)
> +       if (btp && btp->bt_awu_max > mp->m_sb.sb_blocksize)
>                  return max_pow_of_two_factor(g->blocks);
>          return rounddown_pow_of_two(g->blocks);
> }
> 
> -----8<-----
> 
> 
> You are advocating something like this (to solve the issue below), right?
> 

Hmm, this might fix the issue but I still don't understand why we have hardware 
atomics check while determining SW atomics limit. Am I missing something?

> note: that we still need to ensure that bt_awu_min <= sb_blocksize to get HW 
> atomics at all (so should keep a check for bt_awu_min in 
> xfs_calc_group_awu_max() or similar)
> 
>>
--
Pankaj