Re: [PATCH 4/4] block: add large command support

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Apr 17 2008, FUJITA Tomonori wrote:
> On Wed, 16 Apr 2008 12:08:25 +0300
> Boaz Harrosh <bharrosh@panasas.com> wrote:
> 
> > On Wed, Apr 16 2008 at 11:33 +0300, Jens Axboe <jens.axboe@oracle.com> wrote:
> > > On Wed, Apr 16 2008, FUJITA Tomonori wrote:
> > >> On Wed, 16 Apr 2008 00:50:54 +0200
> > >> Bartlomiej Zolnierkiewicz <bzolnier@gmail.com> wrote:
> > >>
> > >>> Hi,
> > >>>
> > >>> On Monday 14 April 2008, Jens Axboe wrote:
> > >>>> On Mon, Apr 14 2008, FUJITA Tomonori wrote:
> > >>>>> This patch changes rq->cmd from the static array to a pointer to
> > >>>>> support large commands.
> > >>>>>
> > >>>>> We rarely handle large commands. So for optimization, a struct request
> > >>>>> still has a static array for a command. rq_init sets rq->cmd pointer
> > >>>>> to the static array.
> > >>>>>
> > >>>>> Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
> > >>>>> Cc: Jens Axboe <jens.axboe@oracle.com>
> > >>>>> ---
> > >>>>>  block/blk-core.c       |    1 +
> > >>>>>  drivers/ide/ide-io.c   |    1 +
> > >>>>>  include/linux/blkdev.h |   12 ++++++++++--
> > >>>>>  3 files changed, 12 insertions(+), 2 deletions(-)
> > >>>>>
> > >>>>> diff --git a/block/blk-core.c b/block/blk-core.c
> > >>>>> index 6669238..6f0968f 100644
> > >>>>> --- a/block/blk-core.c
> > >>>>> +++ b/block/blk-core.c
> > >>>>> @@ -132,6 +132,7 @@ void rq_init(struct request_queue *q, struct request *rq)
> > >>>>>  	rq->errors = 0;
> > >>>>>  	rq->ref_count = 1;
> > >>>>>  	rq->cmd_len = 0;
> > >>>>> +	rq->cmd = rq->__cmd;
> > >>>>>  	memset(rq->cmd, 0, BLK_MAX_CDB);
> > >>>>>  	rq->data_len = 0;
> > >>>>>  	rq->extra_len = 0;
> > >>>>> diff --git a/drivers/ide/ide-io.c b/drivers/ide/ide-io.c
> > >>>>> index 7153796..bac5ea1 100644
> > >>>>> --- a/drivers/ide/ide-io.c
> > >>>>> +++ b/drivers/ide/ide-io.c
> > >>>>> @@ -1595,6 +1595,7 @@ void ide_init_drive_cmd (struct request *rq)
> > >>>>>  {
> > >>>>>  	memset(rq, 0, sizeof(*rq));
> > >>>>>  	rq->ref_count = 1;
> > >>>>> +	rq->cmd = rq->__cmd;
> > >>>>>  }
> > >>> Tomo, some more changes are needed:
> > >>>
> > >>> Please think about all _static_/dynamic allocations of 'struct request'
> > >>> used together with REQ_TYPE_SPECIAL etc., i.e.
> > >> I think that using struct request allocated statically is wrong from
> > >> the perspective of the block layer design, that is, you always need to
> > >> use blk_get_request. I think that except ide, everyone does.
> > >>
> > >> I try to convert ide to use blk_get_request properly if you want.
> > > 
> > > That would be best, but the on-stack allocation has the benefit that
> > > it'll always work. So until we can completely get rid of that, lets just
> > > make it a hard rule that ANY rq allocation MUST call rq_init(). It's a
> > > lot saner than doing a memset() anyway.
> > > 
> > 
> > Just a patch that I had for ages since the bad old request bidi times,
> > perhaps is also good today. (rebased to for-2.6.26 branch)
> > ---
> > From: Boaz Harrosh <bharrosh@panasas.com>
> > Date: Wed, 16 Apr 2008 12:05:33 +0300
> > Subject: [PATCH] Initialize all members of struct request in rq_init
> > 
> > Before, every member added/removed from struct request would entitle a change
> > to rq_init, for initialization. Now all members are default to zero and only
> > the none zero members are specifically initialized.
> > 
> > Users that need requests on the stack or pre-allocated, must call rq_init()
> > before use.
> > 
> > Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
> > ---
> >  block/blk-core.c |   22 ++--------------------
> >  1 files changed, 2 insertions(+), 20 deletions(-)
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index 6f0968f..3f4c563 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -113,36 +113,18 @@ EXPORT_SYMBOL(blk_get_backing_dev_info);
> >   */
> >  void rq_init(struct request_queue *q, struct request *rq)
> >  {
> > +	memset(rq, 0, sizeof(*rq));
> 
> Hmm, rq_init comment says:
> 
> /*
>  * We can't just memset() the structure, since the allocation path
>  * already stored some information in the request.
>  */
> 
> I think that we can't initialize rq->cmd_flags here.

That is correct, the patch wont work as-is. The principle of clearing
every member except block internal is sound and should be applied,
though.

-- 
Jens Axboe