[dm-devel] Re: fragmented i/o with 2.6.31?

[Jun'ichi Nomura]

Mike Snitzer wrote:
> On Fri, Sep 18 2009 at  2:00am -0400,
> Martin K. Petersen <martin.petersen at oracle.com> wrote:
> 
>>>>>>> "Mike" == Mike Snitzer <snitzer at redhat.com> writes:
>>>>>  	blk_set_default_limits(limits);
>>>>> + limits->max_sectors = 0;
>>>>> + limits->max_hw_sectors = 0;
>> Mike> Seems like we may want some common variant in block even though
>> Mike> I'm not aware of other block drivers that would benefit...
>>
>> Mike> But I'll defer to Martin and/or Jens on whether these helpers are
>> Mike> fine to stay in dm-table.c or should be worked into blk-settings.c
>>
>> In the pre-topology days we set max_sectors to SAFE_MAX_SECTORS upon
>> creation of a queue.  This is an old ATA-ism that's been around for a
>> ages.
>>
>> Ideally we'd simply nuke it and drivers that really needed to lower the
>> bar would explicitly call blk_queue_max_sectors().  However, I'm afraid
>> to change the default because I'm sure there are legacy drivers lurking
>> somewhere that depend on it.
>>
>> Seeing as blk_set_default_limits() is mostly aimed at stacking drivers I
>> think I'd prefer moving SAFE_MAX_SECTORS back to blk_queue_make_request
>> and then set max_sectors and max_hw_sectors to 0 in default_limits.
>>
>> Would that work for you guys?
> 
> So you're referring to fact that this commit removed
> blk_queue_max_sectors(q, SAFE_MAX_SECTORS) from blk_queue_make_request:
> http://git.kernel.org/linus/e475bba2
> 
> I think I like your proposal.  But, to clarify things further, are you
> saying:
> 
> By moving SAFE_MAX_SECTORS back to blk_queue_make_request (after its
> existing call to blk_set_default_limits right?) and having
> blk_set_default_limits set max_sectors and max_hw_sectors to 0:
> 
> DM will be free to establish the proper limit stacking because the DM
> limits are not derived from the queue's default limits?  Because the DM
> device limits are just stacked and copied to the queue, some background
> for those following along:
> 
> DM's actual stacking of limits takes place when the DM table is
> translated to the DM device's final queue (at table resume time), see:
> http://git.kernel.org/linus/754c5fc7e
> 
> drivers/md/dm.c:dm_swap_table() calls dm_calculate_queue_limits() to
> stack the limits.
> 
> drivers/md/dm.c:__bind() sets the DM device's queue_limits via
> dm_table_set_restrictions()
> 
> drivers/md/dm-table.c:dm_table_set_restrictions() simply copies the
> queue_limits established by DM's stacking with:
>         /*                                                                                                        
>          * Copy table's limits to the DM device's request_queue                                                                                                        
>          */
>         q->limits = *limits;
> 
> Now coming full circle:
> AFAIK the only piece I'm missing is how/where your proposed changes will
> account for the need to establish SAFE_MAX_SECTORS _after_ the stacking
> of queue_limits: IFF max_sectors and max_hw_sectors are still 0 (like
> Jun'ichi did in DM with the 2nd patch posted).
> 
> But I don't pretend to have this all sorted out in my head.  I could
> easily be missing some other piece(s) implicit in your proposal.
> 
> Maybe an RFC patch that illustrates your thinking would help further this
> discussion?

I just sent out revised patchset:

[PATCH 1/2] dm: Set safe default max_sectors for targets with no 
underlying device
https://www.redhat.com/archives/dm-devel/2009-September/msg00203.html

[PATCH 2/2] block: blk_set_default_limits sets 0 to max_sectors
https://www.redhat.com/archives/dm-devel/2009-September/msg00205.html


But I wonder better fix might be to provide blk_queue_copy_limits()
to replace this in dm-table.c:

 >         q->limits = *limits;

where blk_queue_copy_limits() looks like this:

void blk_queue_copy_limits(struct request_queue *q, struct queue_limits 
*lim)
{
	q->limits = *limits;

	/* fix-up bad values */
	if (q->limits.max_sectors == 0 || q->limits.max_hw_sectors == 0)
		blk_queue_max_sectors(q, SAFE_MAX_SECTORS);
}

so that block/blk-settings.c has full-control on default value
and dm don't need to care about the magic 'SAFE_MAX_SECTORS'.

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation