linux-kernel - Re: [PATCH] block: add a bi

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Date:	Wed, 22 Jul 2015 15:59:46 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Christoph Hellwig <hch@....de>
CC:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Neil Brown <neilb@...e.de>, Liu Bo <bo.li.liu@...cle.com>,
	linux-raid@...r.kernel.org, dm-devel@...hat.com,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: add a bi_error field to struct bio

On 07/22/2015 12:51 PM, Jens Axboe wrote:
> On 07/20/2015 07:29 AM, Christoph Hellwig wrote:
>> Currently we have two different ways to signal an I/O error on a BIO:
>>
>>   (1) by clearing the BIO_UPTODATE flag
>>   (2) by returning a Linux errno value to the bi_end_io callback
>>
>> The first one has the drawback of only communicating a single possible
>> error (-EIO), and the second one has the drawback of not beeing
>> persistent
>> when bios are queued up, and are not passed along from child to parent
>> bio in the ever more popular chaining scenario.  Having both mechanisms
>> available has the additional drawback of utterly confusing driver authors
>> and introducing bugs where various I/O submitters only deal with one of
>> them, and the others have to add boilerplate code to deal with both kinds
>> of error returns.
>>
>> So add a new bi_error field to store an errno value directly in struct
>> bio and remove the existing mechanisms to clean all this up.
>
> I think this is a good change, the only part I _really_ dislike is that
> this now bumps a struct bio from 2 cache lines to 3. Have you done any
> perf testing?

One possible solution would be to shrink bi_flags to an unsigned int, no 
problems fitting that in. Then we could stuff bi_error in that (new) 
hole, and we would end up having the same size again.

-- 
Jens Axboe

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