dm: optimize flushes

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@redhat.com>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Suggested-by: Yang Yang <yang.yang@vivo.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 92d18dcdb3e9..33b7a1844ed4 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -160,6 +160,7 @@ int dm_table_create(struct dm_table **result, blk_mode_t mode,
 	t->type = DM_TYPE_NONE;
 	t->mode = mode;
 	t->md = md;
+	t->flush_bypasses_map = true;
 	*result = t;
 	return 0;
 }
@@ -748,6 +749,9 @@ int dm_table_add_target(struct dm_table *t, const char *type,
 	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: