[PATCH v3 0/12] nvmet: FDP bridge for NVMe-oF targets

[Guixin Liu]

Flexible Data Placement (FDP) is defined in the NVMe base specification
and therefore applies to fabrics as well.  An Ethernet Bunch Of
Flash (EBOF) style storage system that exposes its local FDP-capable
NVMe drives to remote hosts over NVMe-oF currently has no way to pass
that capability through: nvmet neither advertises FDP nor translates
or forwards any FDP command, so a host connected through the target
loses the write amplification benefits that the local drives provide.

This series adds an FDP bridge to the NVMe target.  A namespace backed
by an NVMe namespace opts in through the fdp_enable configfs attribute;
the bridge then advertises the FDPS capability, reports the backing FDP
configuration through the feature and log pages, translates the
placement identifiers of write commands to the backing write streams,
and forwards the FDP management commands and log pages to the backing
controller.  The backing geometry is verified at namespace enable time,
so the bridge only comes up when the backing placement identifiers are
identity mapped, which the translation relies on.

The FDP commands are served by the bridge directly instead of relying
on the existing passthru support, for several reasons:

  - Passthru subsystems do not support reservations; the reservation
    commands are rejected and the controller is not advertised as
    reservation capable, because the underlying device cannot
    differentiate the hosts that connect over fabrics.  A regular
    namespace goes through the target's own reservation implementation,
    which is not available to a passthru subsystem.

  - A passthru subsystem is bound to a single backend controller: the
    whole drive, with all of its namespaces, is passed through as one
    unit, and no regular namespaces can be added to the subsystem.  An
    EBOF system needs to compose a storage pool out of individual
    namespaces from multiple backend drives, which only the regular
    namespace path provides.

  - The regular namespace path already handles the target side of
    reservations, ANA reporting and per-namespace configuration; the
    bridge extends that path rather than replacing it.

  - With passthru, the namespace identity of the backend drive is
    exposed to the host as is, and the host would talk to the backend
    controller's FDP state through the passthrough of every command.
    The bridge instead owns the FDP presentation of the exported
    namespace, while the state that the backing controller owns is
    forwarded to it: the reclaim unit handle status hosts read and the
    reclaim unit handle updates they issue are carried out by the
    controller that actually manages the media.

The Linux host side needs no changes: the existing FDP discovery path
(FDPS bit, endurance group identifier, FDP feature, FDP configurations
log page and reclaim unit handle status) works against the bridge
unmodified.

The series is structured as follows:

  1-2   wire structures and block layer helper
  3     nvme_bdev_ops export, used to recognize an NVMe backing device
  4     per-namespace bridge state and configuration
  5     fdp_enable ABI documentation
  6     endurance group identifier in the CS-INDEP identify data
  7     FDPS advertisement
  8     FDP feature (FID 1Dh)
  9     I/O management command forwarding
  10    FDP log pages (LID 20h-23h)
  11    placement identifier translation on the write path
  12    tracepoint for the translation

This was tested with a QEMU FDP backing device through an nvmet-tcp
loopback setup, with persistent reservations enabled on the exported
namespace: the host discovers and registers the backing write streams
over the bridge, writes with placement identifiers land in the reclaim
units of the backend device they select (verified through the reclaim
unit handle status of the backend, for single bio and chained bio
writes alike), and the forwarded log pages match the backend byte for
byte.  Short and offset reads of the configurations log match the full
page, and Set Features on a namespace without FDP accepts the unchanged
disabled configuration.  I/O management receive requests whose NUMD
arithmetic would wrap, zero-length forwards, and requests above the
1 MiB staging bound are rejected with Invalid Field.  A second
namespace without FDP in the same subsystem comes up normally and
reports FDPE clear.  With a second controller registered, the FDP
commands are served while a write exclusive reservation is held,
writes from the non-holder conflict, and preempt-and-abort completes
with I/O management commands in flight.

Changes since v2:

  https://lore.kernel.org/all/20261023080251.2606443-1-kanie@linux.alibaba.com/

  - Take the namespace reference of the FDP commands under
    NVMET_NS_IO_LIVE with percpu_ref_tryget_live_rcu(), closing a
    use-after-free against a concurrently disabled namespace.
  - Report FDPE clear from Get Features for namespaces without FDP,
    so that a subsystem mixing bridged and regular namespaces does
    not fail the host's per-namespace discovery; Set Features accepts
    that same disabled configuration as a no-op.
  - Evaluate the I/O management receive transfer length in 64 bits
    instead of letting the 32-bit NUMD arithmetic wrap, bound all
    staged FDP transfers to 1 MiB, and reject zero-length forwards.
  - Re-encode NUMD of the forwarded log pages from the staged transfer
    length instead of carrying over the host supplied value, and clear
    the reserved metadata field of the forwarded commands.
  - Fall through to the persistent reservation access checks for the
    I/O management commands instead of returning before them.
  - Require the backing reclaim unit handle count to match the write
    streams exactly instead of accepting a larger count.
  - Read the backing FDP configurations log at enable time so the
    advertised handle types and the 64-bit reclaim unit nominal size
    are the backing device's own values, selected by the active
    FDPCIDX rather than by descriptor order, and validate every
    descriptor before dereferencing it.
  - Serve short reads and nonzero Log Page Offsets of the
    configurations log as windows of the complete page.
  - Run the FDP command handlers from a workqueue so that lpfc cannot
    sleep in hardirq context.
  - Track the FDP capability as a per-subsystem count, incremented at
    namespace enable and decremented at namespace disable.
  - Advertise the I/O management commands in the command-effects log
    only when the subsystem actually bridges FDP, and set error_loc
    for invalid FDP feature requests.
  - Document fdp_enable in the ABI file.

Changes since v1:

  https://lore.kernel.org/all/20261019033624.402337-1-kanie@linux.alibaba.com/

  - Verify the transfer length encoded in the I/O management commands
    against the length the host supplied, and do not carry the host
    data pointer over to the backing command.
  - Assign the translated write stream to every bio of a write, not
    just the first one.
  - Run the namespace lookup of the FDP commands under RCU.
  - Report the FDP feature in the Features log page under the same
    per-subsystem condition as the FDPS bit.
  - Only emit the write translation tracepoint for writes that carry
    the Data Placement Directive.
  - Dispatch the I/O management commands after the persistent
    reservation checks.
  - Report backing command failures during the namespace enable as
    errno, and clean up the backend state when the enable fails.
  - Drop the Assisted-by: LLM tag from all commits.

Guixin Liu (12):
  nvme: add the FDP wire structures for the I/O management send command
  block: add bdev_write_stream_granularity()
  nvme: export nvme_bdev_ops
  nvmet: add per-namespace FDP bridge state and configuration
  nvmet: document the fdp_enable configfs attribute
  nvmet: report the Endurance Group Identifier in the CS-INDEP identify
    data
  nvmet: advertise the FDP capability
  nvmet: implement the FDP feature
  nvmet: forward the FDP I/O management commands to the backing device
  nvmet: serve the FDP log pages
  nvmet: translate placement identifiers of writes to write streams
  nvmet: add an FDP write translation tracepoint

 Documentation/ABI/stable/configfs-nvmet |  13 +
 drivers/nvme/host/core.c                |   1 +
 drivers/nvme/target/Kconfig             |  14 +
 drivers/nvme/target/Makefile            |   1 +
 drivers/nvme/target/admin-cmd.c         |  45 +-
 drivers/nvme/target/configfs.c          |  32 ++
 drivers/nvme/target/core.c              |  43 +-
 drivers/nvme/target/fdp.c               | 682 ++++++++++++++++++++++++
 drivers/nvme/target/io-cmd-bdev.c       |  15 +
 drivers/nvme/target/nvmet.h             |  91 ++++
 drivers/nvme/target/trace.h             |  23 +
 include/linux/blkdev.h                  |   8 +
 include/linux/nvme.h                    |  24 +
 13 files changed, 987 insertions(+), 5 deletions(-)
 create mode 100644 drivers/nvme/target/fdp.c

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2.43.7