[dm-devel] block: Fix a WRITE SAME BUG_ON

[Mike Snitzer]

On Sat, Jan 26 2019 at  6:17am -0500,
John Dorminy <jdorminy at redhat.com> wrote:

> Hi. I have read a bit of DM code and spent an hour reviewing this... I
> didn't get to the point of knowing what the right fix for the problem
> is, and I may have a wrong understanding, but I have two thoughts
> about the patch:
> 
> I don't think this is the right solution for two reasons:
> 
> In the first place, if it's an LVM-only issue, we should fix it only
> for device-mapper devices. If this is the right way to fix it,
> possibly the way to do that would be to change DM calls to
> blk_queue_max_write_same_sectors() to only set the max sectors to more
> than 0 if and only if the logical block sizes match.

There is no way this is specific to lvm (or DM).  It may _seem_ that way
because lvm/dm are in the business of creating stacked devices --
whereby exposing users to blk_stack_limits().

I'll have a closer look at this issue, hopefully tomorrow, but Zhang
Xiaoxu's proposed fix looks bogus to me.  Not disputing there is an
issue, just feels like a different fix is needed.

Mike