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

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

On 10/2/2026 10:45 AM, John Garry wrote:
> On 10/1/26 17:30, Pankaj Raghav wrote:
>>>
>>> ----->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?
> 
> ok, I suppose that it (i.e. whether bt_awu_max > sb_blocksize) should not 
> determine CoW-based atomics limits, but it should determine HW limits (in opt max).
> 

Could you take a look the patches again and see if it makes sense? We already 
take into account the HW limits while calculating opt. So the new function 
xfs_calc_group_awu_align_max() checks for alignment constraints because of agsize.

Without patches:
[nix-shell:~]# trace-cmd report
cpus=16
mount-617   [009]  1184.719159: xfs_calc_atomic_write_unit_max: dev 259:2 ag 
max_write 131072 max_ioend 4096 max_gsize 2 awu_max 2


[nix-shell:~]# ./statx /media/test/hello.txt
/media/test/hello.txt:
   stx_atomic_write_unit_min:            16384
   stx_atomic_write_unit_max:            32768
   stx_atomic_write_unit_max_opt:        16384
   stx_atomic_write_segments_max:        1

With patches:

[nix-shell:~]# trace-cmd report
cpus=16
mount-598   [005]   319.692000: xfs_calc_atomic_write_unit_max: dev 259:2 ag 
max_write 131072 max_ioend 4096 max_gsize 33554432 awu_max 4096

[nix-shell:~]# ./statx /media/test/hello.txt
/media/test/hello.txt:
   stx_atomic_write_unit_min:            16384
   stx_atomic_write_unit_max:            67108864
   stx_atomic_write_unit_max_opt:        16384
   stx_atomic_write_segments_max:        1
>>
>>> 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) 
>