Re: [PATCH] dm-delay: advertise flush support

[Benjamin Marzinski <bmarzins@redhat.com>]

On Fri, Aug 14, 2026 at 03:35:25PM -0400, Benjamin Marzinski wrote:
> On Fri, Aug 14, 2026 at 12:58:12PM +0800, Matthias Goergens wrote:
> > 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).
> > 
> 
> The code seems fine, but could you please change the commit message.
> It's not true that "Anyone who configures a flush delay on dm-delay
> silently measures nothing". dm-delay doesn't *force* the dm device to
> support flushes. It works just like most of the dm targets: linear,
> stripe, raid, etc. If an underlying device sets BLK_FEAT_WRITE_CACHE or
> BLK_FEAT_FUA, the dm device will as well (see blk_stack_limits, called
> by dm_set_device_limits). I assume if you look at
> /sys/block/<underlying_disk>/queue/write_cache, you see "write through".
>

Actually, it might be better to only set ti->flush_supported if
dc->flush.delay is non-zero, so that if the user doesn't want a
flush delay, dm-delay behaves like a linear target for flushes.

-Ben

> -Ben
> 
> > 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,