Re: [PATCH v3 05/13] generic/1226: Add atomic write test using fio crc check verifier

John Garry
2025-07-28T14:01:14Z

On 28/07/2025 14:35, Ojaswin Mujoo wrote:
>> We guarantee that the write is committed all-or-nothing, but do rely on
>> userspace not issuing racing atomic writes or racing regular writes.
>>
>> I can easily change this, as I mentioned, but I am not convinced that it is
>> a must.
> Purely from a design point of view, I feel we are breaking atomicity and
> hence we should serialize or just stop userspace from doing this (which
> is a bit extreme).

If you check the man page description of RWF_ATOMIC, it does not mention
serialization. The user should conclude that usual direct IO rules
apply, i.e. userspace is responsible for serializing.

>
> I know userspace should ideally not do overwriting atomic writes but if
> it is something we are allowing (which we do) then it is
> kernel's responsibility to ensure atomicity. Sure we can penalize them
> by serializing the writes but not by tearing it.
>
> With that reasoning, I don't think the test should accomodate for this
> particular scenario.

I can send a patch to the community for xfs (to provide serialization),
like I showed earlier, to get opinion.

Thanks,
John