On Wed, 10 Sep 2025, Bart Van Assche wrote:
> The dm core splits REQ_PREFLUSH bios that have data into two bios.
> First, a REQ_PREFLUSH bio with no data is submitted to all underlying
> dm devices. Next, the REQ_PREFLUSH flag is cleared and the same bio is
> resubmitted. This approach is essential if there are multiple underlying
> devices to provide correct REQ_PREFLUSH semantics.
>
> Splitting a bio into an empty flush bio and a non-flush data bio is
> not necessary if there is only a single underlying device. Hence this
> patch that does not split REQ_PREFLUSH bios if there is only one
> underlying device.
>
> This patch preserves the order of REQ_PREFLUSH writes if there is only
> one underlying device and if one or more write bios have been queued
> past the REQ_PREFLUSH bio before the REQ_PREFLUSH bio is processed.
>
> Cc: Mike Snitzer <[email protected]>
> Cc: Damien Le Moal <[email protected]>
> Signed-off-by: Bart Van Assche <[email protected]>
> ---
>
> Changes compared to v1:
> - Made the patch description more detailed.
> - Removed the reference to write pipelining from the patch description.
Hi
I think that the problem here is that not all targets handle a PREFLUSH
bio with data (for example, dm-integrity doesn't handle it correctly; it
assumes that the PREFLUSH bio is empty).
I suggest that the logic should be changed to test that
"t->flush_bypasses_map == true" (that will rule out targets that don't
support flush optimization) and "dm_table_get_devices returns just one
device" - if both of these conditions are true, you can send the PREFLUSH
bio with data to the one device that dm_table_get_devices returned.
It will also optimize the case when you have multiple dm-linear targets
with just one underlying device.
Mikulas
