Re: [PATCH 06/14] block: lift setting the readahead size into the block layer

[Christoph Hellwig <hch@lst.de>]

On Thu, Sep 10, 2020 at 01:15:41PM -0400, Mike Snitzer wrote:
> > I'll move it to blk_register_queue, which should work just fine.
> 
> That'll work for initial DM table load as part of DM device creation
> (dm_setup_md_queue).  But it won't account for DM table reloads that
> might change underlying devices on a live DM device (done using
> __bind).
> 
> Both dm_setup_md_queue() and __bind() call dm_table_set_restrictions()
> to set/update queue_limits.  It feels like __bind() will need to call a
> new block helper to set/update parts of queue_limits (e.g. ra_pages and
> io_pages).
> 
> Any chance you're open to factoring out that block function as an
> exported symbol for use by blk_register_queue() and code like DM's
> __bind()?

I agree with the problem statement.  OTOH adding an exported helper
for two trivial assignments seems a little silly..

For now I'll just keep the open coded ->io_pages assignment in
dm.  Note that dm doesn't currently update the ->ra_pages value
based on the underlying devices, so an incremental patch to do that
might be useful as well.