linux-kernel - Re: [PATCH v2] block: do not artificially constrain max

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Message-ID: <20120709225725.GA1487@redhat.com>
Date:	Mon, 9 Jul 2012 18:57:25 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	"Chauhan, Vijay" <Vijay.Chauhan@...app.com>
Cc:	axboe@...nel.dk, "Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-kernel@...r.kernel.org,
	"Stankey, Robert" <Robert.Stankey@...app.com>,
	"Moger, Babu" <Babu.Moger@...app.com>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>
Subject: Re: [PATCH v2] block: do not artificially constrain max_sectors for
 stacking drivers

On Mon, Jul 09 2012 at 10:57am -0400,
Mike Snitzer <snitzer@...hat.com> wrote:

> blk_set_stacking_limits() is intended to allow stacking drivers to build
> up the limits of the stacked device based on the underlying devices'
> limits.  But in the case of 'max_sectors' the default of
> BLK_DEF_MAX_SECTORS (1024) doesn't allow the stacking driver to inherit
> a max_sectors larger than 1024.
> 
> It is now clear that this artificial limit is getting in the way so
> change blk_set_stacking_limits's max_sectors to UINT_MAX (which allows
> stacking drivers like dm-multipath to inherit 'max_sectors' from the
> underlying paths).
> 
> blk_limits_max_hw_sectors() must allow stacking drivers to not have
> max_sectors set to BLK_DEF_MAX_SECTORS as a side-effect.  Move that
> historic constraint to blk_queue_max_hw_sectors().
> 
> Reported-by: Vijay Chauhan <vijay.chauhan@...app.com>
> Signed-off-by: Mike Snitzer <snitzer@...hat.com>
> Cc: Martin K. Petersen <martin.petersen@...cle.com>
> ---
>  block/blk-settings.c |    8 ++++----
>  1 files changed, 4 insertions(+), 4 deletions(-)
> 
> v2: tweak blk_limits_max_hw_sectors and blk_queue_max_hw_sectors

As it happens, v2's changes to blk_limits_max_hw_sectors and
blk_queue_max_hw_sectors are not strictly required in order for existing
stacking drivers to have have an unconstrained max_sectors.  Dropping
those changes also allows for consistency across both block functions.

So I'd be happy if v1 were to be staged for 3.6.  NetApp: it would be
great if you could confirm that v1 does in fact address the max_sectors
issue you reported.

Thanks,
Mike
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