Re: [PATCH 4/5] nvme: implement multipath access to nvme subsystems

This patch adds native multipath support to the nvme driver.  For each
namespace we create only single block device node, which can be used
to access that namespace through any of the controllers that refer to it.
The gendisk for each controllers path to the name space still exists
inside the kernel, but is hidden from userspace.  The character device
nodes are still available on a per-controller basis.  A new link from
the sysfs directory for the subsystem allows to find all controllers
for a given subsystem.

Currently we will always send I/O to the first available path, this will
be changed once the NVMe Asynchronous Namespace Access (ANA) TP is
ratified and implemented, at which point we will look at the ANA state
for each namespace.  Another possibility that was prototyped is to
use the path that is closes to the submitting NUMA code, which will be
mostly interesting for PCI, but might also be useful for RDMA or FC
transports in the future.  There is not plan to implement round robin
or I/O service time path selectors, as those are not scalable with
the performance rates provided by NVMe.

The multipath device will go away once all paths to it disappear,
any delay to keep it alive needs to be implemented at the controller
level.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/nvme/host/Kconfig     |   9 ++
 drivers/nvme/host/Makefile    |   1 +
 drivers/nvme/host/core.c      | 133 +++++++++++++++++++---
 drivers/nvme/host/multipath.c | 255 ++++++++++++++++++++++++++++++++++++++++++
 drivers/nvme/host/nvme.h      |  57 ++++++++++
 5 files changed, 440 insertions(+), 15 deletions(-)
 create mode 100644 drivers/nvme/host/multipath.c

diff --git a/drivers/nvme/host/Kconfig b/drivers/nvme/host/Kconfig
index 46d6cb1e03bd..45886800a1df 100644
--- a/drivers/nvme/host/Kconfig
+++ b/drivers/nvme/host/Kconfig

@@ -13,6 +13,15 @@ config BLK_DEV_NVME
 	  To compile this driver as a module, choose M here: the
 	  module will be called nvme.
 
+config NVME_MULTIPATH
+	bool "NVMe multipath support"
+	depends on NVME_CORE
+	---help---
+	   This option enables support for multipath access to NVMe
+	   subsystems.  If this option is enabled only a single
+	   /dev/nvneXnY device will show up for each NVMe namespaces,
+	   even if it is accessible through multiple controllers.
+
 config NVME_FABRICS
 	tristate