[PATCH v2] dm-delay: advertise flush support when a flush delay is configured

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

dm-delay has a flush delay class: the 9-argument table form takes
<flush_device> <flush_offset> <flush_delay>, delay_map() routes
REQ_PREFLUSH bios to it, num_flush_bios is 1, and
Documentation/admin-guide/device-mapper/delay.rst documents it. But
the target never sets ti->flush_supported, so the dm device only
supports flushes when an underlying device stacks BLK_FEAT_WRITE_CACHE
into its limits. Over a write-through device (write_cache "write
through") it does not: submit_bio_noacct() strips REQ_PREFLUSH and
REQ_FUA before any bio reaches the target, and a configured flush
delay measures nothing.

Set ti->flush_supported when dc->flush.delay is non-zero. A configured
flush delay then always sees its flushes. Without one, dm-delay keeps
stacking flush support from the underlying device like the other
targets, so an undelayed dm-delay behaves like linear for flushes.
As for any table with flush support, the dm queue then also
advertises FUA; delay_map() keeps routing REQ_FUA writes to the
write class, and only REQ_PREFLUSH goes to the flush class.

Verified in a VM with dm-delay over a write-through virtio-scsi disk,
using a 9-argument table with write delay 0 and a flush delay of 200
or 800 ms. Without this change the dm queue reports write_cache "write
through", fua 0, and five 4 KiB dd oflag=dsync writes to the dm device
complete in 0-1 ms. With it the queue reports "write back", fua 1, and
the same writes take 204-210 ms and 812-880 ms. Zero-delay tables over
the write-through disk and over a write-back disk behave the same with
and without the change and keep the underlying queue's settings.

Suggested-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
---
Changes since v1:
- Set flush_supported only when dc->flush.delay is non-zero, so an
  undelayed dm-delay behaves like linear for flushes (Ben).
- Commit message describes only the write-through case; otherwise dm
  stacks flush support from the underlying device (Ben).
- Dropped the claim that FUA writes reach the wire: tracing the
  underlying disk shows a plain data write and a REQ_PREFLUSH bio.
- Re-measured with write delay 0 so the timing isolates the flush class.

 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
index 4864671c2222f..96bb430215a32 100644
--- a/drivers/md/dm-delay.c
+++ b/drivers/md/dm-delay.c
@@ -301,6 +301,7 @@ static int delay_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	}
 
 	ti->num_flush_bios = 1;
+	ti->flush_supported = dc->flush.delay != 0;
 	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 int delay_iterate_devices(struct dm_target *ti,
 
 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,
-- 
2.55.0