On Fri, Aug 31 2007, Kiyoshi Ueda wrote:
> Hello,
>
> This set of patches changes request completion interface
> between device drivers and block layer to 1 step procedure
> from current 2 step procedures using end_that_request_{first/chunk}
> and end_that_request_last().
>
> This change allows request-based multipath to hook in before
> completing each chunk of request, check errors for it and
> retry it using another path if error is detected.
>
> Summaries of each patch are below:
> 1/7: add new request completion interface, blk_end_request()
> 2/7: add some macros to get the size of request in bytes
> 3/7: convert normal drivers to use blk_end_request()
> 4/7: convert odd drivers like cciss/cpqarray/xsysace to use
> blk_end_request()
> 5/7: convert ide-cd (cdrom_newpc_intr) to use blk_end_request()
> 6/7: remove/unexport no longer needed end_that_request_*
> 7/7: change rq->end_io to cover request completion as a whole
>
> I have tested the patch on two machines, ia64+QLA1280+QLA2200
> and x86_64+SATA+IDE-CDROM.
> I can't test other device drivers for which I don't have hardware.
> So testing help and any comments would be very much appreciated.
>
> The interface change causes code modifications of *ALL DEVICE DRIVERS*
> which are using end_that_request_{first/chunk/last} to complete request.
> But it should not affect the behavior.
>
> Please review and apply if no problem.
> This patch-set should be applied on top of 2.6.23-rc3-mm1.
>
>
> BACKGROUND
> ==========
> The patch is necessary to allow device stacking at request level,
> that is request-based device-mapper multipath.
> Currently, device-mapper is implemented as a stacking block device
> at BIO level. OTOH, request-based DM will stack at request level to
> allow better multipathing decision.
> To allow device stacking at request level, the completion procedure
> need to provide a hook for it.
> For example, dm-multipath has to check errors and retry with other
> paths if necessary before returning the I/O result to upper layer.
> struct request has 'end_io' hook currently. But it's called at
> the very late stage of completion handling where the I/O result
> is already returned to the upper layer.
> So we need something here.
>
> The first approach to hook in completion of each chunk of request
> was adding a new rq->end_io_first() hook and calling it on the top
> of __end_that_request_first().
> - http://marc.theaimsgroup.com/?l=linux-scsi&m=115520444515914&w=2
> - http://marc.theaimsgroup.com/?l=linux-kernel&m=116656637425880&w=2
> However, Jens pointed out that redesigning rq->end_io() as a full
> completion handler would be better:
>
> On Thu, 21 Dec 2006 08:49:47 +0100, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:
> > Ok, I see what you are getting at. The current ->end_io() is called when
> > the request has fully completed, you want notification for each chunk
> > potentially completed.
> >
> > I think a better design here would be to use ->end_io() as the full
> > completion handler, similar to how bio->bi_end_io() works. A request
> > originating from __make_request() would set something ala:
> .....
> > instead of calling the functions manually. That would allow you to get
> > notification right at the beginning and do what you need, without adding
> > a special hook for this.
>
> I thought his comment was reasonable.
> So I modified the patches based on his suggestion.
>
>
> WHAT IS CHANGED
> ===============
> The change is basically illustlated by the following pseudo code:
>
> [Before]
> if (end_that_request_{first/chunk} succeeds) { <-- completes bios
> <do something driver specific>
> end_that_request_last() <-- calls end_io()
> <the request is free from the driver>
> } else {
> <the request was incomplete, retry for leftover or ignoring>
> }
>
> [After]
> if (blk_end_request() succeeds) { <-- calls end_io(), completes bios
> <the request is free from the driver>
> } else {
> <the request was incomplete, retry for leftover or ignoring>
> }
>
>
> In detail, request completion procedures are changed like below.
>
> [Before]
> o 2 steps completion using end_that_request_{first/chunk}
> and end_that_request_last().
> o Device drivers have ownership of a request until they
> call end_that_request_last().
> o rq->end_io() is called at the last stage of
> end_that_request_last() for some block layer codes need
> specific request handling when completing it.
>
> [After]
> o 1 step completion using blk_end_request().
> (end_that_request_* are no longer used from device drivers.)
> o Device drivers give over ownership of a request
> when calling blk_end_request().
> If it returns 0, the request is completed.
> If it returns 1, the request isn't completed and
> the ownership is returned to the device driver again.
> o rq->end_io() is called at the top of blk_end_request() to
> allow to hook all parts of request completion.
> Existing users of rq->end_io() must be changed to do
> all parts of request completion.
>
>
> EXAMPLE CODE
> ============
> Request-based Device-mapper multipath patch-set is attached as appendix,
> although it still needs some work and isn't ready for review.
> It checks error of a request and retries the request using other paths
> if error is detected, before completing bios in the request.
> (See clone_end_request() in appendix#1.)
This looks good, thanks for following up on this! I've replied with
comments on changes for the core bits, the interface I quite agree with.
So if you fix up the things I ask for, I'll merge this up.
--
Jens Axboe
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