[PATCHv5 0/2] block, nvme: removing virtual boundary mask reliance

Keith Busch
2025-10-14T15:05:13Z

From: Keith Busch <kbusch@kernel.org>

Previous version here:

https://lore.kernel.org/linux-block/20251007175245.3898972-1-kbusch@meta.com/

The purpose is to allow optimization decisions to happen per IO, and
flexibility to utilize unaligned buffers for hardware that supports it.

The virtual boundary that NVMe uses provides specific guarantees about
the data alignment, but that might not be large enough for some CPU
architectures to take advantage of even if an applications uses aligned
data buffers that could use it.

At the same time, the virtual boundary prevents the driver from directly
using memory in ways the hardware may be capable of accessing. This
creates unnecessary needs on applications to double buffer their data
into a more restrictive virtually contiguous format.

This patch series provides an efficient way to track segment boundary
gaps per-IO so that the optimizations can be decided per-IO. This
provides flexibility to use all hardware to their abilities beyond what
the virtual boundary mask can provide.

Note, abuse of this capability may result in worse performance compared
to the bounce buffer solutions. Sending a bunch of tiny vectors for one
IO incurs significant protocol overhead, so while this patch set allows
you to do that, I recommend that you don't. We can't enforce a minimum
size though because vectors may straddle pages with only a few words in
the first and/or last pages, which we do need to support.

Changes from v4:

* Keep the same lowest-set-bit representation in the request as the
bio; provide a helper to turn it into a mask

* Open-code the bvec gaps calculation since the helper is being removed

* Additional code comments

* Keeping the virt boundary unchanged for the loop target for now. Only
pci, tcp, and fc are not reporting such a boundary.

Keith Busch (2):
block: accumulate memory segment gaps per bio
nvme: remove virtual boundary for sgl capable devices

block/bio.c | 1 +
block/blk-map.c | 3 +++
block/blk-merge.c | 39 ++++++++++++++++++++++++++++++++++---
block/blk-mq-dma.c | 3 +--
block/blk-mq.c | 6 ++++++
drivers/nvme/host/apple.c | 1 +
drivers/nvme/host/core.c | 10 +++++-----
drivers/nvme/host/fabrics.h | 6 ++++++
drivers/nvme/host/fc.c | 1 +
drivers/nvme/host/nvme.h | 7 +++++++
drivers/nvme/host/pci.c | 28 +++++++++++++++++++++++---
drivers/nvme/host/rdma.c | 1 +
drivers/nvme/host/tcp.c | 1 +
drivers/nvme/target/loop.c | 1 +
include/linux/bio.h | 2 ++
include/linux/blk-mq.h | 16 +++++++++++++++
include/linux/blk_types.h | 12 ++++++++++++
17 files changed, 125 insertions(+), 13 deletions(-)

--
2.47.3