[dm-devel] How can need dm use limits.max_hw_sectors from the bdev?

[Mike Snitzer]

On Sun, Jan 17 2016 at  1:34am -0500,
Eric Wheeler <dm-devel at lists.ewheeler.net> wrote:

> Hello all,
> 
> I'm writing a trivial dm target and hitting errors like this:
>    io too big device loop0 (2048 > 255)
> 
> which looks like the problem described here:
> 	https://bugzilla.kernel.org/show_bug.cgi?id=9401#c3
> 
> and is is consistent:
> 	cat /sys/block/loop0/queue/max_hw_sectors_kb 
> 	127
> 	# cat /sys/block/dm-4/queue/max_hw_sectors_kb 
> 	2147483647
> 
> By tracing from sysfs it looks like I need to do something like this when 
> called from the target constructor (dm_ctr_fn target_type.ctr) function:
> 
>     target->table->md->queue->limits.max_hw_sectors =
>         priv->dm_dev_bdev->bdev->bd_queue->limits.max_hw_sectors;
> 
> but I cannot because `struct md` and `struct mapped_device` are opaque
> which makes me think there is a better (correct) way to do this.  Is there a way
> to change my target device's max_hw_sectors value to 127?
> 
> Is there some other generic solution?

You'll first want to implement the .iterate_devices hook.  This alone
should enable DM core to leverage the block layer's blk_stack_limits()
infrastructure to stack up your top-level device's queue_limits.

And if you'd like to override your device's queue_limits beyond what is
stacked up from the underlying device(s) you can implement the .io_hints
hook in your target.

> Would late bio splitting solve this?

Maybe, if by solve this you mean: the block core won't error any more.
But you're better off stacking underlying devices' queue_limits like I
detailed above.

Mike