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

[Benjamin Marzinski <bmarzins@redhat.com>]

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".

-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,