On Thu, Sep 10 2020 at  5:28am -0400,
Christoph Hellwig [off-list ref] wrote:

On Wed, Sep 02, 2020 at 12:20:07PM -0400, Mike Snitzer wrote:

quoted

On Wed, Sep 02 2020 at 11:11am -0400,
Christoph Hellwig [off-list ref] wrote:

quoted

On Wed, Aug 26, 2020 at 06:07:38PM -0400, Mike Snitzer wrote:

quoted

On Sun, Jul 26 2020 at 11:03am -0400,
Christoph Hellwig [off-list ref] wrote:

quoted

Drivers shouldn't really mess with the readahead size, as that is a VM
concept.  Instead set it based on the optimal I/O size by lifting the
algorithm from the md driver when registering the disk.  Also set
bdi->io_pages there as well by applying the same scheme based on
max_sectors.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/blk-settings.c         |  5 ++---
 block/blk-sysfs.c            |  1 -
 block/genhd.c                | 13 +++++++++++--
 drivers/block/aoe/aoeblk.c   |  2 --
 drivers/block/drbd/drbd_nl.c | 12 +-----------
 drivers/md/bcache/super.c    |  4 ----
 drivers/md/dm-table.c        |  3 ---
 drivers/md/raid0.c           | 16 ----------------
 drivers/md/raid10.c          | 24 +-----------------------
 drivers/md/raid5.c           | 13 +------------
 10 files changed, 16 insertions(+), 77 deletions(-)

In general these changes need a solid audit relative to stacking
drivers.  That is, the limits stacking methods (blk_stack_limits)
vs lower level allocation methods (__device_add_disk).

You optimized for lowlevel __device_add_disk establishing the bdi's
ra_pages and io_pages.  That is at the beginning of disk allocation,
well before any build up of stacking driver's queue_io_opt() -- which
was previously done in disk_stack_limits or driver specific methods
(e.g. dm_table_set_restrictions) that are called _after_ all the limits
stacking occurs.

By inverting the setting of the bdi's ra_pages and io_pages to be done
so early in __device_add_disk it'll break properly setting these values
for at least DM afaict.

ra_pages never got inherited by stacking drivers, check it by modifying
it on an underlying device and then creating a trivial dm or md one.

Sure, not saying that it did.  But if the goal is to set ra_pages based
on io_opt then to do that correctly on stacking drivers it must be done
in terms of limits stacking right?  Or at least done at a location that
is after the limits stacking has occurred?  So should DM just open-code
setting ra_pages like it did for io_pages?

Because setting ra_pages in __device_add_disk() is way too early for DM
-- given it uses device_add_disk_no_queue_reg via add_disk_no_queue_reg
at DM device creation (before stacking all underlying devices' limits).

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()?

Thanks,
Mike