linux-kernel - Re: [PATCH 2/4] readv.2: Document RWF

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Date:   Wed, 4 Oct 2023 09:47:24 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     Bart Van Assche <bvanassche@....org>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Cc:     martin.petersen@...cle.com, djwong@...nel.org, david@...morbit.com,
        himanshu.madhani@...cle.com
Subject: Re: [PATCH 2/4] readv.2: Document RWF_ATOMIC flag

On 03/10/2023 20:25, Bart Van Assche wrote:
> On 9/29/23 02:37, John Garry wrote:
>> +.BR RWF_ATOMIC " (since Linux 6.7)"
>> +Allows block-based filesystems to indicate that write operations will 
>> be issued
>> +with torn-write protection. Torn-write protection means that for a 
>> power or any
>> +other hardware failure, all or none of the data from the write will 
>> be stored,
>> +but never a mix of old and new data. This flag is meaningful only for
>> +.BR pwritev2 (),
>> +and its effect applies only to the data range written by the system 
>> call.
>> +The total write length must be power-of-2 and must be sized between
>> +stx_atomic_write_unit_min and stx_atomic_write_unit_max, both 
>> inclusive. The
>> +write must be at a natural offset within the file with respect to the 
>> total
>> +write length. Torn-write protection only works with
>> +.B O_DIRECT
>> +flag, i.e. buffered writes are not supported. To guarantee 
>> consistency from
>> +the write between a file's in-core state with the storage device,
> 
> It seems wrong to me to start the first sentence with "Allows". Atomic
> behavior should be mandatory if RWF_ATOMIC has been set.

Yes, I agree that this has been poorly worded. Flag RWF_ATOMIC does not 
indicate anything. I will fix it.

> 
> Additionally, shouldn't it be documented what value will be stored in
> errno if the atomic write has been rejected?

So I was treating all atomic writes errors which don't follow the 
"rules" as low-level I/O errors, which is -EIO. However, yes, I can 
document this. Further to that, based on description of an error for 
O_DIRECT, which is to return -EINVAL for misaligned, I think that 
-EINVAL may be better for any atomic write rule violations. OK?

Thanks,
John