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Date: Thu, 23 May 2024 19:46:25 +0200 (CEST)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Mike Snitzer <snitzer@...nel.org>
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: [PATCH v2] dm: optimize flushes
Hi
Here I'm resending the patch, with more comments and explanations added.
Mikulas
From: Mikulas Patocka <mpatocka@...hat.com>
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_bypasses_map - 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_bypasses_map,
flush_bypasses_map on the table is set. __send_empty_flush tests if the
table has flush_bypasses_map - and if it has, no flush bios are sent to
the targets via the "map" method and the list dm_table->devices is
iterated and the flush bios are sent to each member of the list.
Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
Suggested-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 | 54 +++++++++++++++++++++++++++++++-----------
include/linux/device-mapper.h | 15 +++++++++++
6 files changed, 64 insertions(+), 15 deletions(-)
Index: linux-2.6/drivers/md/dm-core.h
===================================================================
--- linux-2.6.orig/drivers/md/dm-core.h 2024-05-23 19:00:00.000000000 +0200
+++ linux-2.6/drivers/md/dm-core.h 2024-05-23 19:00:00.000000000 +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_bypasses_map" set */
+ bool flush_bypasses_map: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 2024-05-23 19:00:00.000000000 +0200
+++ linux-2.6/drivers/md/dm-linear.c 2024-05-23 19:00:00.000000000 +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_bypasses_map = true;
ti->private = lc;
return 0;
Index: linux-2.6/drivers/md/dm-stripe.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-stripe.c 2024-05-23 19:00:00.000000000 +0200
+++ linux-2.6/drivers/md/dm-stripe.c 2024-05-23 19:00:00.000000000 +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_bypasses_map = 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 2024-05-23 19:00:00.000000000 +0200
+++ linux-2.6/drivers/md/dm-table.c 2024-05-23 19:00:00.000000000 +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_bypasses_map = true;
*result = t;
return 0;
}
@@ -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_bypasses_map)
+ t->flush_bypasses_map = false;
+
return 0;
bad:
Index: linux-2.6/include/linux/device-mapper.h
===================================================================
--- linux-2.6.orig/include/linux/device-mapper.h 2024-05-23 19:00:00.000000000 +0200
+++ linux-2.6/include/linux/device-mapper.h 2024-05-23 19:18:01.000000000 +0200
@@ -397,6 +397,21 @@ 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. If all the targets in
+ * a table have flush_bypasses_map set, the dm core will not send
+ * flushes to the targets via a ->map method. It will iterate over
+ * dm_table->devices and send flushes to the devices directly. This
+ * optimization reduces the number of flushes being sent when multiple
+ * targets in a table use the same underlying device.
+ *
+ * This optimization may be enabled on targets that just pass the
+ * flushes to the underlying devices without performing any other
+ * actions on the flush request. Currently, dm-linear and dm-stripe
+ * support it.
+ */
+ bool flush_bypasses_map:1;
};
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 2024-05-23 19:00:00.000000000 +0200
+++ linux-2.6/drivers/md/dm.c 2024-05-23 19:31:14.000000000 +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);
@@ -1566,17 +1566,43 @@ 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_bypasses_map) {
+ 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 optimization 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;
+ /*
+ * Note that the structure dm_target_io is not
+ * associated with any target (because the device may be
+ * used by multiple targets), so we set tio->ti = NULL.
+ * We must check for NULL in the I/O processing path, to
+ * avoid NULL pointer dereference.
+ */
+ 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);
+ }
}
/*
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