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

From: Mike Snitzer
Date: Wed Aug 26 2020 - 18:08:02 EST

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On Sun, Jul 26 2020 at 11:03am -0400,
Christoph Hellwig <hch@xxxxxx> 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@xxxxxx>
> ---
> 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@xxxxxxxxxx
> https://www.redhat.com/mailman/listinfo/dm-devel

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