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Date:   Wed, 26 Aug 2020 18:07:38 -0400
From:   Mike Snitzer <snitzer@redhat.com>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-raid@vger.kernel.org,
        Hans de Goede <hdegoede@redhat.com>,
        Minchan Kim <minchan@kernel.org>,
        Richard Weinberger <richard@nod.at>,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        Song Liu <song@kernel.org>, dm-devel@redhat.com,
        linux-mtd@lists.infradead.org, cgroups@vger.kernel.org,
        drbd-dev@tron.linbit.com, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, martin.petersen@oracle.com
Subject: Re: [PATCH 06/14] block: lift setting the readahead size into the
 block layer
Message-ID: <20200826220737.GA25613@redhat.com>
References: <20200726150333.305527-1-hch@lst.de>
 <20200726150333.305527-7-hch@lst.de>
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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.

Mike


> 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);
>  
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 7dda709f3ccb6f..ce418d9128a0b2 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -245,7 +245,6 @@ queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
>  
>  	spin_lock_irq(&q->queue_lock);
>  	q->limits.max_sectors = max_sectors_kb << 1;
> -	q->backing_dev_info->io_pages = max_sectors_kb >> (PAGE_SHIFT - 10);
>  	spin_unlock_irq(&q->queue_lock);
>  
>  	return ret;
> diff --git a/block/genhd.c b/block/genhd.c
> index 8b1e9f48957cb5..097d4e4bc0b8a2 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -775,6 +775,7 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
>  			      const struct attribute_group **groups,
>  			      bool register_queue)
>  {
> +	struct request_queue *q = disk->queue;
>  	dev_t devt;
>  	int retval;
>  
> @@ -785,7 +786,7 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
>  	 * registration.
>  	 */
>  	if (register_queue)
> -		elevator_init_mq(disk->queue);
> +		elevator_init_mq(q);
>  
>  	/* minors == 0 indicates to use ext devt from part0 and should
>  	 * be accompanied with EXT_DEVT flag.  Make sure all
> @@ -815,10 +816,18 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
>  		disk->flags |= GENHD_FL_SUPPRESS_PARTITION_INFO;
>  		disk->flags |= GENHD_FL_NO_PART_SCAN;
>  	} else {
> -		struct backing_dev_info *bdi = disk->queue->backing_dev_info;
> +		struct backing_dev_info *bdi = q->backing_dev_info;
>  		struct device *dev = disk_to_dev(disk);
>  		int ret;
>  
> +		/*
> +		 * For read-ahead of large files to be effective, we need to
> +		 * readahead at least twice the optimal I/O size.
> +		 */
> +		bdi->ra_pages = max(queue_io_opt(q) * 2 / PAGE_SIZE,
> +				    VM_READAHEAD_PAGES);
> +		bdi->io_pages = queue_max_sectors(q) >> (PAGE_SHIFT - 9);
> +
>  		/* Register BDI before referencing it from bdev */
>  		dev->devt = devt;
>  		ret = bdi_register(bdi, "%u:%u", MAJOR(devt), MINOR(devt));
> diff --git a/drivers/block/aoe/aoeblk.c b/drivers/block/aoe/aoeblk.c
> index 5ca7216e9e01f3..89b33b402b4e52 100644
> --- a/drivers/block/aoe/aoeblk.c
> +++ b/drivers/block/aoe/aoeblk.c
> @@ -347,7 +347,6 @@ aoeblk_gdalloc(void *vp)
>  	mempool_t *mp;
>  	struct request_queue *q;
>  	struct blk_mq_tag_set *set;
> -	enum { KB = 1024, MB = KB * KB, READ_AHEAD = 2 * MB, };
>  	ulong flags;
>  	int late = 0;
>  	int err;
> @@ -407,7 +406,6 @@ aoeblk_gdalloc(void *vp)
>  	WARN_ON(d->gd);
>  	WARN_ON(d->flags & DEVFL_UP);
>  	blk_queue_max_hw_sectors(q, BLK_DEF_MAX_SECTORS);
> -	q->backing_dev_info->ra_pages = READ_AHEAD / PAGE_SIZE;
>  	d->bufpool = mp;
>  	d->blkq = gd->queue = q;
>  	q->queuedata = d;
> diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
> index 650372ee2c7822..212bf711fb6b41 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);
>  }
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 9e45faa054b6f4..9d3f0711be030f 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1367,10 +1367,6 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
>  	if (ret)
>  		return ret;
>  
> -	dc->disk.disk->queue->backing_dev_info->ra_pages =
> -		max(dc->disk.disk->queue->backing_dev_info->ra_pages,
> -		    q->backing_dev_info->ra_pages);
> -
>  	atomic_set(&dc->io_errors, 0);
>  	dc->io_disable = false;
>  	dc->error_limit = DEFAULT_CACHED_DEV_ERROR_LIMIT;
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index aac4c31cfc8498..324a42ed2f8894 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -1924,9 +1924,6 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
>  		q->nr_zones = blkdev_nr_zones(t->md->disk);
>  	}
>  #endif
> -
> -	/* Allow reads to exceed readahead limits */
> -	q->backing_dev_info->io_pages = limits->max_sectors >> (PAGE_SHIFT - 9);
>  }
>  
>  unsigned int dm_table_get_num_targets(struct dm_table *t)
> diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
> index f54a449f97aa79..aa2d7279176880 100644
> --- a/drivers/md/raid0.c
> +++ b/drivers/md/raid0.c
> @@ -410,22 +410,6 @@ static int raid0_run(struct mddev *mddev)
>  		 mdname(mddev),
>  		 (unsigned long long)mddev->array_sectors);
>  
> -	if (mddev->queue) {
> -		/* calculate the max read-ahead size.
> -		 * For read-ahead of large files to be effective, we need to
> -		 * readahead at least twice a whole stripe. i.e. number of devices
> -		 * multiplied by chunk size times 2.
> -		 * If an individual device has an ra_pages greater than the
> -		 * chunk size, then we will not drive that device as hard as it
> -		 * wants.  We consider this a configuration error: a larger
> -		 * chunksize should be used in that case.
> -		 */
> -		int stripe = mddev->raid_disks *
> -			(mddev->chunk_sectors << 9) / PAGE_SIZE;
> -		if (mddev->queue->backing_dev_info->ra_pages < 2* stripe)
> -			mddev->queue->backing_dev_info->ra_pages = 2* stripe;
> -	}
> -
>  	dump_zones(mddev);
>  
>  	ret = md_integrity_register(mddev);
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 9f88ff9bdee437..23d15acbf457d4 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -3865,19 +3865,6 @@ static int raid10_run(struct mddev *mddev)
>  	mddev->resync_max_sectors = size;
>  	set_bit(MD_FAILFAST_SUPPORTED, &mddev->flags);
>  
> -	if (mddev->queue) {
> -		int stripe = conf->geo.raid_disks *
> -			((mddev->chunk_sectors << 9) / PAGE_SIZE);
> -
> -		/* Calculate max read-ahead size.
> -		 * We need to readahead at least twice a whole stripe....
> -		 * maybe...
> -		 */
> -		stripe /= conf->geo.near_copies;
> -		if (mddev->queue->backing_dev_info->ra_pages < 2 * stripe)
> -			mddev->queue->backing_dev_info->ra_pages = 2 * stripe;
> -	}
> -
>  	if (md_integrity_register(mddev))
>  		goto out_free_conf;
>  
> @@ -4715,17 +4702,8 @@ static void end_reshape(struct r10conf *conf)
>  	conf->reshape_safe = MaxSector;
>  	spin_unlock_irq(&conf->device_lock);
>  
> -	/* read-ahead size must cover two whole stripes, which is
> -	 * 2 * (datadisks) * chunksize where 'n' is the number of raid devices
> -	 */
> -	if (conf->mddev->queue) {
> -		int stripe = conf->geo.raid_disks *
> -			((conf->mddev->chunk_sectors << 9) / PAGE_SIZE);
> -		stripe /= conf->geo.near_copies;
> -		if (conf->mddev->queue->backing_dev_info->ra_pages < 2 * stripe)
> -			conf->mddev->queue->backing_dev_info->ra_pages = 2 * stripe;
> +	if (conf->mddev->queue)
>  		raid10_set_io_opt(conf);
> -	}
>  	conf->fullsync = 0;
>  }
>  
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 68e41ce3ca75cc..415ce3cc155698 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -7413,8 +7413,6 @@ static int raid5_run(struct mddev *mddev)
>  		int data_disks = conf->previous_raid_disks - conf->max_degraded;
>  		int stripe = data_disks *
>  			((mddev->chunk_sectors << 9) / PAGE_SIZE);
> -		if (mddev->queue->backing_dev_info->ra_pages < 2 * stripe)
> -			mddev->queue->backing_dev_info->ra_pages = 2 * stripe;
>  
>  		chunk_size = mddev->chunk_sectors << 9;
>  		blk_queue_io_min(mddev->queue, chunk_size);
> @@ -8002,17 +8000,8 @@ static void end_reshape(struct r5conf *conf)
>  		spin_unlock_irq(&conf->device_lock);
>  		wake_up(&conf->wait_for_overlap);
>  
> -		/* read-ahead size must cover two whole stripes, which is
> -		 * 2 * (datadisks) * chunksize where 'n' is the number of raid devices
> -		 */
> -		if (conf->mddev->queue) {
> -			int data_disks = conf->raid_disks - conf->max_degraded;
> -			int stripe = data_disks * ((conf->chunk_sectors << 9)
> -						   / PAGE_SIZE);
> -			if (conf->mddev->queue->backing_dev_info->ra_pages < 2 * stripe)
> -				conf->mddev->queue->backing_dev_info->ra_pages = 2 * stripe;
> +		if (conf->mddev->queue)
>  			raid5_set_io_opt(conf);
> -		}
>  	}
>  }
>  
> -- 
> 2.27.0
> 
> --
> dm-devel mailing list
> dm-devel@redhat.com
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