[PATCH] dm-delay: advertise flush support

[Matthias Goergens <matthias.goergens@gmail.com>]

dm-delay parses and reports a flush delay class (the 9-argument table
form's <flush_device> <flush_offset> <flush_delay>), delay_map() routes
REQ_PREFLUSH bios to it, the target sets num_flush_bios = 1, and
Documentation/admin-guide/device-mapper/delay.rst documents flush
delays. But the target never sets ti->flush_supported, so
dm_table_supports_flush() is false: the block layer strips REQ_PREFLUSH
as a no-op before the bio ever reaches the target (blk_insert_flush()),
REQ_FUA is stripped too, and the flush delay class is dead code. Anyone
who configures a flush delay on dm-delay (e.g. to model slow-flush
devices in tests) silently measures nothing.

Set ti->flush_supported = true so the flush delay class is actually
reachable. dm core clones empty flushes with
REQ_OP_WRITE | REQ_PREFLUSH | REQ_SYNC (__send_empty_flush), which
delay_map() already routes to the flush class, and delayed completion
is already handled for every class by the delay worker.

Verified with dm-delay over a virtio-scsi disk: without this change
REQ_PREFLUSH completes immediately and REQ_FUA writes are stripped by
the block layer; with it, flushes take the configured delay and FUA
writes reach the wire (measured as device-time per MB at D_f = 200 ms
and 800 ms flush delays).

Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
---
 drivers/md/dm-delay.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-delay.c b/drivers/md/dm-delay.c
--- a/drivers/md/dm-delay.c
+++ b/drivers/md/dm-delay.c
@@ -301,6 +301,7 @@
 	}

 	ti->num_flush_bios = 1;
+	ti->flush_supported = true;
 	ti->num_discard_bios = 1;
 	ti->accounts_remapped_io = true;
 	ti->per_io_data_size = sizeof(struct dm_delay_info);
@@ -451,7 +452,7 @@

 static struct target_type delay_target = {
 	.name	     = "delay",
-	.version     = {1, 5, 0},
+	.version     = {1, 5, 1},
 	.features    = DM_TARGET_PASSES_INTEGRITY | DM_TARGET_ZONED_HM,
 	.module      = THIS_MODULE,
 	.ctr	     = delay_ctr,