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Message-ID: <Zk4Y6DMgK71UuoKd@kernel.org>
Date: Wed, 22 May 2024 12:10:16 -0400
From: Mike Snitzer <snitzer@...nel.org>
To: Mikulas Patocka <mpatocka@...hat.com>
Cc: Benjamin Marzinski <bmarzins@...hat.com>,
Yang Yang <yang.yang@...o.com>, Alasdair Kergon <agk@...hat.com>,
dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: dm: optimize flushes
On Thu, May 16, 2024 at 10:49:55PM +0200, Mikulas Patocka wrote:
> Device mapper sends flush bios to all the targets and the targets send it
> to the underlying device. That may be inefficient, for example if a table
> contains 10 linear targets pointing to the same physical device, then
> device mapper would send 10 flush bios to that device - despite the fact
> that only one bio would be sufficient.
>
> This commit optimizes the flush behavior. It introduces a per-target
> variable flush_pass_around - it is set when the target supports flush
> optimization - currently, the dm-linear and dm-stripe targets support it.
> When all the targets in a table have flush_pass_around, flush_pass_around
> on the table is set. __send_empty_flush tests if the table has
> flush_pass_around - and if it has, no flush bios are sent to the targets
> and the list dm_table->devices is iterated and the flush bios are sent to
> each member of the list.
What does "pass around" mean? Seems like an awkward name for this.
(Naming can be hard, I don't have better suggestions at the moment.)
> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> Reported-by: Yang Yang <yang.yang@...o.com>
>
> ---
> drivers/md/dm-core.h | 4 ++-
> drivers/md/dm-linear.c | 1
> drivers/md/dm-stripe.c | 1
> drivers/md/dm-table.c | 4 +++
> drivers/md/dm.c | 47 +++++++++++++++++++++++++++++-------------
> include/linux/device-mapper.h | 5 ++++
> 6 files changed, 47 insertions(+), 15 deletions(-)
>
> Index: linux-2.6/drivers/md/dm-core.h
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-core.h 2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm-core.h 2024-05-15 16:56:49.000000000 +0200
> @@ -206,7 +206,9 @@ struct dm_table {
>
> bool integrity_supported:1;
> bool singleton:1;
> - unsigned integrity_added:1;
> + bool integrity_added:1;
> + /* set if all the targets in the table have "flush_pass_around" set */
> + bool flush_pass_around:1;
>
> /*
> * Indicates the rw permissions for the new logical device. This
> Index: linux-2.6/drivers/md/dm-linear.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-linear.c 2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm-linear.c 2024-05-15 16:56:49.000000000 +0200
> @@ -62,6 +62,7 @@ static int linear_ctr(struct dm_target *
> ti->num_discard_bios = 1;
> ti->num_secure_erase_bios = 1;
> ti->num_write_zeroes_bios = 1;
> + ti->flush_pass_around = true;
> ti->private = lc;
> return 0;
>
> Index: linux-2.6/drivers/md/dm-stripe.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-stripe.c 2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm-stripe.c 2024-05-15 16:56:49.000000000 +0200
> @@ -157,6 +157,7 @@ static int stripe_ctr(struct dm_target *
> ti->num_discard_bios = stripes;
> ti->num_secure_erase_bios = stripes;
> ti->num_write_zeroes_bios = stripes;
> + ti->flush_pass_around = true;
>
> sc->chunk_size = chunk_size;
> if (chunk_size & (chunk_size - 1))
> Index: linux-2.6/drivers/md/dm-table.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-table.c 2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm-table.c 2024-05-15 16:56:49.000000000 +0200
> @@ -160,6 +160,7 @@ int dm_table_create(struct dm_table **re
> t->type = DM_TYPE_NONE;
> t->mode = mode;
> t->md = md;
> + t->flush_pass_around = 1;
> *result = t;
> return 0;
> }
Should be: t->flush_pass_around = true;
> @@ -738,6 +739,9 @@ int dm_table_add_target(struct dm_table
> if (ti->limit_swap_bios && !static_key_enabled(&swap_bios_enabled.key))
> static_branch_enable(&swap_bios_enabled);
>
> + if (!ti->flush_pass_around)
> + t->flush_pass_around = false;
> +
> return 0;
>
> bad:
> Index: linux-2.6/include/linux/device-mapper.h
> ===================================================================
> --- linux-2.6.orig/include/linux/device-mapper.h 2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/include/linux/device-mapper.h 2024-05-15 16:56:49.000000000 +0200
> @@ -397,6 +397,11 @@ struct dm_target {
> * bio_set_dev(). NOTE: ideally a target should _not_ need this.
> */
> bool needs_bio_set_dev:1;
> +
> + /*
> + * Set if the target supports flush optimization
> + */
> + bool flush_pass_around:1;
> };
How does a developer _know_ if a target can set this flag? Please
elaborate on the requirements in this code comment.
>
> void *dm_per_bio_data(struct bio *bio, size_t data_size);
> Index: linux-2.6/drivers/md/dm.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm.c 2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm.c 2024-05-16 20:06:32.000000000 +0200
> @@ -645,7 +645,7 @@ static struct bio *alloc_tio(struct clon
>
> /* Set default bdev, but target must bio_set_dev() before issuing IO */
> clone->bi_bdev = md->disk->part0;
> - if (unlikely(ti->needs_bio_set_dev))
> + if (likely(ti != NULL) && unlikely(ti->needs_bio_set_dev))
> bio_set_dev(clone, md->disk->part0);
>
> if (len) {
> @@ -1107,7 +1107,7 @@ static void clone_endio(struct bio *bio)
> blk_status_t error = bio->bi_status;
> struct dm_target_io *tio = clone_to_tio(bio);
> struct dm_target *ti = tio->ti;
> - dm_endio_fn endio = ti->type->end_io;
> + dm_endio_fn endio = likely(ti != NULL) ? ti->type->end_io : NULL;
> struct dm_io *io = tio->io;
> struct mapped_device *md = io->md;
>
> @@ -1154,7 +1154,7 @@ static void clone_endio(struct bio *bio)
> }
>
> if (static_branch_unlikely(&swap_bios_enabled) &&
> - unlikely(swap_bios_limit(ti, bio)))
> + likely(ti != NULL) && unlikely(swap_bios_limit(ti, bio)))
> up(&md->swap_bios_semaphore);
>
> free_tio(bio);
What is it about this commit that makes it important to verify ti
isn't NULL in the above 3 hunks?
Should these NULL checks be factored out as a separate fix?
Or can these hunks be dropped?
> @@ -1566,17 +1566,36 @@ static void __send_empty_flush(struct cl
> ci->sector_count = 0;
> ci->io->tio.clone.bi_iter.bi_size = 0;
>
> - for (unsigned int i = 0; i < t->num_targets; i++) {
> - unsigned int bios;
> - struct dm_target *ti = dm_table_get_target(t, i);
> -
> - if (unlikely(ti->num_flush_bios == 0))
> - continue;
> -
> - atomic_add(ti->num_flush_bios, &ci->io->io_count);
> - bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
> - NULL, GFP_NOWAIT);
> - atomic_sub(ti->num_flush_bios - bios, &ci->io->io_count);
> + if (!t->flush_pass_around) {
> + for (unsigned int i = 0; i < t->num_targets; i++) {
> + unsigned int bios;
> + struct dm_target *ti = dm_table_get_target(t, i);
> +
> + if (unlikely(ti->num_flush_bios == 0))
> + continue;
> +
> + atomic_add(ti->num_flush_bios, &ci->io->io_count);
> + bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
> + NULL, GFP_NOWAIT);
> + atomic_sub(ti->num_flush_bios - bios, &ci->io->io_count);
> + }
> + } else {
> + /*
> + * Note that there's no need to grab t->devices_lock here
> + * because the targets that support flush pass-around don't
> + * modify the list of devices.
> + */
> + struct list_head *devices = dm_table_get_devices(t);
> + unsigned int len = 0;
> + struct dm_dev_internal *dd;
> + list_for_each_entry(dd, devices, list) {
> + struct bio *clone;
> + clone = alloc_tio(ci, NULL, 0, &len, GFP_NOIO);
> + atomic_add(1, &ci->io->io_count);
> + bio_set_dev(clone, dd->dm_dev->bdev);
> + clone->bi_end_io = clone_endio;
> + dm_submit_bio_remap(clone, NULL);
> + }
> }
>
> /*
>
>
Still missing what "pass-around" is meant to convey given that you
aren't passing around the same flush... you're cloning a new flush and
issuing one per device. Probably worth explaining that's what you
mean by "flush_pass_around" (both in commit header and elaborate in
code)?
Also, you're issuing a flush to _all_ devices in a table. Not just
the data devices. .iterate_devices returns only the data devices.
If/when there is a need to extend this feature to targets that have
metadata devices (e.g. dm-thin, cache, etc): would it make sense to
filter out non-data devices (by stepping through each target in the
table and using iterate_devices)?
Mike
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