Re: [PATCH 07/13] block: lift setting the readahead size into the block layer

From: Jan Kara <jack@suse.cz>
To: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>, Song Liu <song@kernel.org>,
	Hans de Goede <hdegoede@redhat.com>, Coly Li <colyli@suse.de>,
	Richard Weinberger <richard@nod.at>,
	Minchan Kim <minchan@kernel.org>,
	Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
	Justin Sanders <justin@coraid.com>,
	linux-mtd@lists.infradead.org, dm-devel@redhat.com,
	linux-block@vger.kernel.org, linux-bcache@vger.kernel.org,
	linux-kernel@vger.kernel.org, drbd-dev@lists.linbit.com,
	linux-raid@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, cgroups@vger.kernel.org
Subject: Re: [PATCH 07/13] block: lift setting the readahead size into the block layer
Date: Tue, 22 Sep 2020 11:13:14 +0200	[thread overview]
Message-ID: <20200922091314.GD16464@quack2.suse.cz> (raw)
In-Reply-To: <20200921080734.452759-8-hch@lst.de>

On Mon 21-09-20 10:07:28, Christoph Hellwig 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>
> Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
...
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 76a7e03bcd6cac..01049e9b998f1d 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -452,6 +452,8 @@ EXPORT_SYMBOL(blk_limits_io_opt);
>  void blk_queue_io_opt(struct request_queue *q, unsigned int opt)
>  {
>  	blk_limits_io_opt(&q->limits, opt);
> +	q->backing_dev_info->ra_pages =
> +		max(queue_io_opt(q) * 2 / PAGE_SIZE, VM_READAHEAD_PAGES);
>  }
>  EXPORT_SYMBOL(blk_queue_io_opt);
>  
> @@ -628,9 +630,6 @@ void disk_stack_limits(struct gendisk *disk, struct block_device *bdev,
>  		printk(KERN_NOTICE "%s: Warning: Device %s is misaligned\n",
>  		       top, bottom);
>  	}
> -
> -	t->backing_dev_info->io_pages =
> -		t->limits.max_sectors >> (PAGE_SHIFT - 9);
>  }
>  EXPORT_SYMBOL(disk_stack_limits);

One thing I've noticed is that blk_stack_limits() does not use
blk_queue_io_opt() to set new optimal limit. That means that ra_pages won't
be updated for the new queue. E.g. your DRDB change below will result in
ra_pages not being properly updated AFAICT.

Similarly it isn't clear to me how io_pages would get updated after
blk_stack_limits() updates max_hw_sectors...

Otherwise the patch looks good.

								Honza

> diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
> index aaff5bde391506..f8fb1c9b1bb6c1 100644
> --- a/drivers/block/drbd/drbd_nl.c
> +++ b/drivers/block/drbd/drbd_nl.c
> @@ -1360,18 +1360,8 @@ static void drbd_setup_queue_param(struct drbd_device *device, struct drbd_backi
>  	decide_on_discard_support(device, q, b, discard_zeroes_if_aligned);
>  	decide_on_write_same_support(device, q, b, o, disable_write_same);
>  
> -	if (b) {
> +	if (b)
>  		blk_stack_limits(&q->limits, &b->limits, 0);
> -
> -		if (q->backing_dev_info->ra_pages !=
> -		    b->backing_dev_info->ra_pages) {
> -			drbd_info(device, "Adjusting my ra_pages to backing device's (%lu -> %lu)\n",
> -				 q->backing_dev_info->ra_pages,
> -				 b->backing_dev_info->ra_pages);
> -			q->backing_dev_info->ra_pages =
> -						b->backing_dev_info->ra_pages;
> -		}
> -	}
>  	fixup_discard_if_not_supported(q);
>  	fixup_write_zeroes(device, q);
>  }
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR