[RFC PATCH 0/2] improve NVMe multipath handling

[Nilay Shroff <nilay@linux.ibm.com>]

Hi,

This patch series introduces improvements to NVMe multipath handling by
refining the removal behavior of the multipath head node and simplifying
configuration options. The idea/POC for this change was originally
proposed by Christoph[1] and Keith[2]. I worked upon their original
idea/POC and implemented this series.

The first patch in the series addresses an issue where the multipath
head node of a PCIe NVMe disk is removed immediately when all disk paths
are lost. This can cause problems in scenarios such as:
- Hot removal and re-addition of a disk.
- Transient PCIe link failures that trigger re-enumeration,
  briefly removing and restoring the disk.

In such cases, premature removal of the head node may result in a device
node name change, requiring applications to reopen device handles if
they were performing I/O during the failure. To mitigate this, we
introduce a delayed removal mechanism. Instead of removing the head node
immediately, the system waits for a configurable timeout, allowing the
disk to recover. If the disk comes back online within this window, the
head node remains unchanged, ensuring uninterrupted workloads.

A new sysfs attribute, delayed_shutdown_sec, allows users to configure
this timeout. By default, it is set to 0 seconds, preserving the
existing behavior unless explicitly changed.

Additionally, please note that this change now always creates head disk
node for all types of NVMe disks (single-ported or multi-ported) as well
as shared/private namespaces, unless the multipath nvme-core module
parameter is explicitly set to false or CONFIG_NVME_MULTIPATH is disabled.

The second patch removes the multipath module parameter parameter from
nvme-core, making native NVMe multipath support explicit. Now with first
patch changes, the multipath head node is always created, even for single-
port NVMe disks when CONFIG_NVME_MULTIPATH is configured. Since this
behavior is now default, the multipath module parameter may no longer be
needed. IMO, the CONFIG_NVME_MULTIPATH (native-multipath) should be the
default and non-native multipath should ideally be deprecated by now,
however I didn't remove CONFIG_NVME_MULTIPATH in this series. So users 
who still prefers non-native multipath can disable CONFIG_NVME_MULTIPATH 
at compile time. Having said that, if everyone agress we may depreacte 
non-native multipath support for NVMe.

These changes should help improve NVMe multipath reliability and simplify
configuration. Feedback and testing are welcome!

PS: Yes I know this RFC is late, but the intention is to get feedback/
suggestion in the upcoming LSF/MM/BPF summit. This might be used as a
reference implementation for discussion. I also saw that we've already
got a timeslot where John is going to talk about removing NVMe multipath
config option. Maybe we could include it in that discussion, if everyone
agress.

Thanks!
--Nilay

Nilay Shroff (2):
  nvme-multipath: introduce delayed removal of the multipath head node
  nvme-multipath: remove multipath module param

 drivers/nvme/host/core.c      |  36 ++++------
 drivers/nvme/host/multipath.c | 127 ++++++++++++++++++++++++++--------
 drivers/nvme/host/nvme.h      |   5 +-
 drivers/nvme/host/sysfs.c     |  13 ++++
 4 files changed, 132 insertions(+), 49 deletions(-)

-- 
2.47.1