[PATCH 0/7] blk_end_request: full I/O completion handler

[Kiyoshi Ueda <k-ueda@ct.jp.nec.com>]

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@oracle.com> 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.)

Thanks,
Kiyoshi Ueda
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