[PATCH v3 0/5] nfs: Modernize Direct I/O path

[Pranjal Shrivastava <praan@google.com>]

Modernize the NFS Direct I/O path as a preparatory step to enable PCI
Peer-to-Peer DMA (P2PDMA) support. Following feedback on the initial
RFC [1], the modernization and architectural changes are split into
different series. Additionally, based on the discussion in the v2 [2]
of this series, the migration of NFS Direct I/O to folios would be
handled in a separate follow-up series.

Currently, NFS O_DIRECT relies on the legacy iov_iter_get_pages_alloc2()
API which does not support the pinning requirements for P2P memory.
The implementation moves NFS to the modern iov_iter_extract_pages() API.

Design
======

1. Pin-Awareness
Standard NFS requests use get_page() and put_page() for memory
management. However, memory extracted via iov_iter_extract_pages()
requires explicit pinning.

Introduce a PG_PINNED flag and a wb_nr_pinned count to struct nfs_page.
This allows the request lifecycle to track ownership of physical pins
and ensure that unpinning is performed only when the I/O is complete.

2. API Migration
Migrate the Direct I/O path to the modern iov_iter_extract_pages()
API. This aligns NFS with the modern extraction model and serves as
the foundation for passing ITER_ALLOW_P2PDMA in a follow-up series.

Upcoming Work / Roadmap
================================

As decided in the RFC [1] & v2 [2], there will be separate series for
P2PDMA Enablement and Migrating NFS Direct I/O to use folios.

This series lays the necessary groundwork for the upcoming work.
Following this, two additional series are planned:

1. Migrating NFS Direct I/O to Folios
A series that introduces and exports helper from iov_iter.c to allow
the nfs_direct_extract_pages() helper introduced here to aggregate
multiple pages into a single large folio-based request, aiming to reduce
the RPC overhead for hugepage I/O.

2. P2PDMA Enablement for NFS
Enabling ITER_ALLOW_P2PDMA for the Direct I/O path and introducing transport-level
negotiation (discovery of P2P-capable RDMA/NVMe devices). This will build
upon the PG_PINNED infrastructure introduced by this series.

Testing
=======
This series has been tested with xfstests [3] on RDMA & TCP transports
by running the quick test suite for each transport vs. version combo:

./check -g quick -s rdma3 -s rdma40 -s rdma41 -s rdma42 -s tcp3 -s tcp40 -s tcp41 -s tcp42

The tests were run before & after applying the series. No regressions were observed. 

The following summary was tabulated via a custom script [4] (on github)
to depict that the tests failing in v1 [5] are now passing.

python3 display.py results/*/check.log

+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+
| testcase     | rdma-sys-3   | rdma-sys-4.0 | rdma-sys-4.1 | rdma-sys-4.2 | tcp-sys-3    | tcp-sys-4.0  | tcp-sys-4.1  | tcp-sys-4.2  |
+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+
| generic/091  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/130  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/139  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/143  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/154  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/155  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/183  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/188  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/190  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/196  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/198  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/203  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/214  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/240  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/263  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/287  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/290  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/292  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/330  | skipped      | skipped      | skipped      | pass         | skipped      | skipped      | skipped      | pass         |
| generic/444  | skipped      | skipped      | skipped      | skipped      | skipped      | skipped      | skipped      | skipped      |
| generic/450  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/451  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/586  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/647  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/708  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/729  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
| generic/760  | pass         | pass         | pass         | pass         | pass         | pass         | pass         | pass         |
+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+

Thanks,
Praan

[1] https://lore.kernel.org/all/20260401194501.2269200-1-praan@google.com/
[2] https://lore.kernel.org/all/ak8-NMsNPOB3zpF-@infradead.org/
[3] https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git
[4] https://github.com/pran005/tools/blob/main/display.py
[5] https://lore.kernel.org/all/29a0511d-5216-46f2-a7e4-9c04ae9b1890@app.fastmail.com/


[v3]
 - Dropped patches that added folio support for NFS Direct I/O
 - Folded requested_bytes accounting in patch 5 due to dropped folio support
 - Rebased on fs-next

[v2] 
 - https://lore.kernel.org/all/20260616134000.2733403-1-praan@google.com/
 - Fix data corruption in nfs_direct_extract_pages() by correctly
   calculating intra-page offsets using offset_in_page().
 - Fix requested_bytes accounting in direct read/write paths to only
   increment after successful RPC scheduling.
 - Add missing kernel-doc descriptions for the @pinned parameter in
   nfs_page_create_from_page() and nfs_page_create_from_folio().
 - Rebase on fs-next/

[v1] https://lore.kernel.org/all/20260603053033.3300318-1-praan@google.com/

Pranjal Shrivastava (5):
  nfs: make nfs_page pin-aware
  nfs: Track number of pinned pages in nfs_page
  nfs: Introduce nfs_release_request_list helper
  nfs: migrate direct I/O to iov_iter_extract_pages
  nfs: introduce nfs_direct_extract_pages helper

 fs/nfs/direct.c          | 143 ++++++++++++++++++++-------------------
 fs/nfs/pagelist.c        |  61 ++++++++++++++---
 fs/nfs/read.c            |   2 +-
 fs/nfs/write.c           |   2 +-
 include/linux/nfs_page.h |   8 ++-
 5 files changed, 134 insertions(+), 82 deletions(-)


base-commit: 98cd7881a2161fe187da5636e1c6cb53d741307a
-- 
2.55.0.229.g6434b31f56-goog