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

From: Christoph Hellwig <hch@lst.de>
To: Mike Snitzer <snitzer@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-raid@vger.kernel.org, martin.petersen@oracle.com,
	Hans de Goede <hdegoede@redhat.com>,
	Richard Weinberger <richard@nod.at>,
	Minchan Kim <minchan@kernel.org>,
	drbd-dev@tron.linbit.com, linux-kernel@vger.kernel.org,
	linux-block@vger.kernel.org, Song Liu <song@kernel.org>,
	dm-devel@redhat.com, linux-mtd@lists.infradead.org,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	cgroups@vger.kernel.org, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 06/14] block: lift setting the readahead size into the block layer
Date: Thu, 10 Sep 2020 11:28:13 +0200	[thread overview]
Message-ID: <20200910092813.GA27229@lst.de> (raw)
In-Reply-To: <20200902162007.GB5513@redhat.com>

On Wed, Sep 02, 2020 at 12:20:07PM -0400, Mike Snitzer wrote:
> On Wed, Sep 02 2020 at 11:11am -0400,
> Christoph Hellwig <hch@lst.de> wrote:
> 
> > On Wed, Aug 26, 2020 at 06:07:38PM -0400, Mike Snitzer wrote:
> > > On Sun, Jul 26 2020 at 11:03am -0400,
> > > Christoph Hellwig <hch@lst.de> wrote:
> > > 
> > > > 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.