[dm-devel] [PATCH 0/8] mutipath: handle nvme:tcp paths better

[Martin Wilck]

On Fri, 2022-10-07 at 12:35 -0500, Benjamin Marzinski wrote:
> nvme:tcp devices set BLK_MQ_F_BLOCKING (they are the only block
> devices
> which multipath supports that do so), meaning that block_mq expects
> that
> they can block at certain points while servicing a request.  However,
> due to the way device-mapper sets up its queue, it is not able to set
> BLK_MQ_F_BLOCKING when it includes paths that set this flag.  Patches
> were written to address this issue but they were rejected upstream
> 
> https://lore.kernel.org/linux-block/YcH%2FE4JNag0QYYAa@infradead.org/T/#t
> 
> The proposed solution was to have multipath use the bio queue_mode
> for
> multipath devices that include nvme:tcp paths.

Request-based dm-multipath is superior to bio-based in multiple ways. I
thought that was consensus. It's sad that we need to make this clumsy
workaround in user space while a much neater kernel-side approach would
be available.

> Multipathd currently doesn't handle queue_mode as well as it could.
> Once
> a multipath device is created, queue_mode cannot be changed, but
> multipath doesn't enforce this. This patchset improves multipath's
> handling of the queue_mode feature, and then makes sure that if a
> multipath device includes a nvme:tcp path, it will have queue_mode
> set to bio.
> 
> Benjamin Marzinski (8):
>   libmultipath: cleanup remove_feature
>   libmultipath: cleanup add_feature
>   multipath tests: tests for adding and removing features
>   libmultipath: fix queue_mode feature handling
>   multipath tests: tests for reconcile_features_with_queue_mode
>   libmultipath: prepare proto_id for use by non-scsi devivces
>   libmultipath: get nvme path transport protocol
>   libmultipath: enforce queue_mode bio for nmve:tcp paths
> 
>  libmultipath/configure.c   |  28 +-
>  libmultipath/discovery.c   |  31 ++-
>  libmultipath/dmparser.c    |   2 +
>  libmultipath/print.c       |   6 +-
>  libmultipath/propsel.c     |  55 ++++
>  libmultipath/structs.c     | 155 +++++------
>  libmultipath/structs.h     |  44 ++-
>  libmultipath/structs_vec.c |   7 +
>  multipath/multipath.conf.5 |  17 +-
>  multipathd/fpin_handlers.c |   2 +-
>  tests/Makefile             |   4 +-
>  tests/features.c           | 549
> +++++++++++++++++++++++++++++++++++++
>  12 files changed, 784 insertions(+), 116 deletions(-)
>  create mode 100644 tests/features.c
> 

For the series: 

Reviewed-by: Martin Wilck <mwilck at suse.com>

I have a few remarks and suggestions for additional improvement which
I'll send as replies to the individual patches.