Re: [PATCH v7 06/15] iomap: Return error code from iomap_write_iter()

[Stefan Roesch <shr@fb.com>]

On 6/6/22 12:18 PM, Matthew Wilcox wrote:
> On Mon, Jun 06, 2022 at 09:39:03AM -0700, Stefan Roesch wrote:
>>
>>
>> On 6/2/22 5:38 AM, Matthew Wilcox wrote:
>>> On Wed, Jun 01, 2022 at 02:01:32PM -0700, Stefan Roesch wrote:
>>>> Change the signature of iomap_write_iter() to return an error code. In
>>>> case we cannot allocate a page in iomap_write_begin(), we will not retry
>>>> the memory alloction in iomap_write_begin().
>>>
>>> loff_t can already represent an error code.  And it's already used like
>>> that.
>>>
>>>> @@ -829,7 +830,8 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>>>>  		length -= status;
>>>>  	} while (iov_iter_count(i) && length);
>>>>  
>>>> -	return written ? written : status;
>>>> +	*processed = written ? written : error;
>>>> +	return error;
>>>
>>> I think the change you really want is:
>>>
>>> 	if (status == -EAGAIN)
>>> 		return -EAGAIN;
>>> 	if (written)
>>> 		return written;
>>> 	return status;
>>>
>>
>> I think the change needs to be:
>>
>> -    return written ? written : status;
>> +    if (status == -EAGAIN) {
>> +        iov_iter_revert(i, written);
>> +        return -EAGAIN;
>> +    }
>> +    if (written)
>> +        return written;
>> +    return status;
> 
> Ah yes, I think you're right.
> 
> Does it work to leave everything the way it is, call back into the
> iomap_write_iter() after having done a short write, get the -EAGAIN at
> that point and pass the already-advanced iterator to the worker thread?
> I haven't looked into the details of how that works, so maybe you just
> can't do that.

With the above change, short writes are handled correctly.