Re: [PATCH for-6.10-rc1] block: fix blk_validate_limits() to properly handle stacked devices

[Mike Snitzer <snitzer@kernel.org>]

On Thu, May 23, 2024 at 05:44:35PM +0200, Christoph Hellwig wrote:
> On Thu, May 23, 2024 at 11:38:21AM -0400, Mike Snitzer wrote:
> > Sure, we could elevate it to blk_validate_limits (and callers) but
> > adding a 'stacking' parameter is more intrusive on an API level.
> > 
> > Best to just update blk_set_stacking_limits() to set a new 'stacking'
> > flag in struct queue_limits, and update blk_stack_limits() to stack
> > that flag up.
> > 
> > I've verified this commit to work and have staged it in linux-next via
> > linux-dm.git's 'for-next', see:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=cedc03d697ff255dd5b600146521434e2e921815
> > 
> > Jens (and obviously: Christoph, Ming and others), I'm happy to send
> > this to Linus tomorrow morning if you could please provide your
> > Reviewed-by or Acked-by.  I'd prefer to keep the intermediate DM fix
> > just to "show the work and testing".
> 
> A stacking flag in the limits is fundamentally wrong, please don't
> do this.

Um, how so?  It serves as a hint to how the limits were constructed.

Reality is, we have stacking block devices that regularly are _not_
accounted for when people make changes to block core queue_limits
code.  That is a serious problem.

Happy to see the need for the 'stacking' flag to go away in time but I
fail to see why it is "fundamentally wrong".

Mike