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

Stefan Roesch
2022-06-06T19:28:33Z

On 6/6/22 12:25 PM, Matthew Wilcox wrote:
> On Mon, Jun 06, 2022 at 12:21:28PM -0700, Stefan Roesch wrote:
>>
>>
>> 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.
>
> Are they though? What about a write that crosses an extent boundary?
> iomap_write_iter() gets called once per extent and I have a feeling that
> you really need to revert the entire write, rather than just the part
> of the write that was to that extent.
>
> Anyway, my question was whether we need to revert or whether the worker
> thread can continue from where we left off.

Without iov_iter_revert() fsx fails with errors in short writes and also my test
program which issues short writes fails.