linux-ext4 - Re: [PATCH v2] iomap: return partial I/O count on error in iomap_dio_bio

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Date:   Wed, 26 Feb 2020 02:12:40 +0000
From:   Damien Le Moal <Damien.LeMoal@....com>
To:     Christoph Hellwig <hch@...radead.org>,
        Ritesh Harjani <riteshh@...ux.ibm.com>
CC:     Goldwyn Rodrigues <rgoldwyn@...e.com>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>
Subject: Re: [PATCH v2] iomap: return partial I/O count on error in
 iomap_dio_bio_actor

On 2020/02/26 5:53, Christoph Hellwig wrote:
> On Fri, Feb 21, 2020 at 10:21:04AM +0530, Ritesh Harjani wrote:
>>>   		if (dio->error) {
>>>   			iov_iter_revert(dio->submit.iter, copied);
>>> -			copied = ret = 0;
>>> +			ret = 0;
>>>   			goto out;
>>>   		}
>>
>> But if I am seeing this correctly, even after there was a dio->error
>> if you return copied > 0, then the loop in iomap_dio_rw will continue
>> for next iteration as well. Until the second time it won't copy
>> anything since dio->error is set and from there I guess it may return
>> 0 which will break the loop.
> 
> In addition to that copied is also iov_iter_reexpand call.  We don't
> really need the re-expand in case of errors, and in fact we also
> have the iov_iter_revert call before jumping out, so this will
> need a little bit more of an audit and properly documented in the
> commit log.
> 
>>
>> Is this the correct flow? Shouldn't the while loop doing
>> iomap_apply in iomap_dio_rw should also break in case of
>> dio->error? Or did I miss anything?
> 
> We'd need something there iff we care about a good number of written
> in case of the error.  Goldwyn, can you explain what you need this
> number for in btrfs?  Maybe with a pointer to the current code base?

Not sure about btrfs, but for zonefs, getting the partial I/O count done for a
failed large dio would also be useful to avoid having to do the error recovery
dance with report zones for getting the current zone write pointer.


-- 
Damien Le Moal
Western Digital Research