Re: [PATCH v2] block: disallow changing max_sectors_kb on a request stacking device

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Nov 07 2016 at  2:32pm -0500,
Jens Axboe <axboe@kernel.dk> wrote:

> On 11/07/2016 12:26 PM, Mike Snitzer wrote:
> >Otherwise users can easily shoot themselves in the foot by creating the
> >situation where the top-level stacked device (e.g. DM multipath) has a
> >larger max_sectors_kb than the underlying device(s).  Which will
> >certainly lead to IO errors due to the "over max size limit" check in
> >blk_cloned_rq_check_limits().
> >
> >This is a crude, yet effective, solution that forces the use of system
> >software (e.g. udev rules or multipathd) to tweak max_sectors_kb of the
> >underlying devices _before_ a layer like DM multipath is layered ontop.
> 
> Maybe I'm missing something, but the code we have in place splits it
> into max sectors for software and hardware. Shouldn't the stacked
> devices have max_hw_sectors capped to what the lower levels support? If
> that was done, we would not have to worry about a user fiddling with
> max_sectors_kb, since it could only be smaller (or equal to) the max
> size of the lower level.

DM multipath just uses blk_stack_limits() to stack limits, which has:

        t->max_sectors = min_not_zero(t->max_sectors, b->max_sectors);
        t->max_hw_sectors = min_not_zero(t->max_hw_sectors, b->max_hw_sectors);
        t->max_dev_sectors = min_not_zero(t->max_dev_sectors, b->max_dev_sectors);

But I assume you realize that.. I'm just missing the relation you're
saying exists, or should exist, between max_hw_sectors and max_sectors
(other than the obvious: max_sectors cannot be greater than
max_hw_sectors) as they relate to stacking.

You're suggesting that when the DM multipath device's limits are stacked
up from the underlying devices: cap the mpath's max_hw_sectors to the
underlying devices' max_sectors?