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Documentation: ublk: address review comments for SHMEM_ZC docs

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- Use "physical pages" instead of "page frame numbers (PFNs)" for
  clarity
- Remove "without any per-I/O overhead" claim from zero-copy
  description
- Add scatter/gather limitation: each I/O's data must be contiguous
  within a single registered buffer

Suggested-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260409133020.3780098-7-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Ming Lei
2026-04-09 21:30:18 +08:00
committed by Jens Axboe
parent 365ea7cc62
commit 289653bb76
1 changed files with 6 additions and 4 deletions
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10
Documentation/block/ublk.rst
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@@ -492,8 +492,8 @@ The ``UBLK_F_SHMEM_ZC`` feature provides an alternative zero-copy path
that works by sharing physical memory pages between the client application
and the ublk server. Unlike the io_uring fixed buffer approach above,
shared memory zero copy does not require io_uring buffer registration
per I/O — instead, it relies on the kernel matching page frame numbers
(PFNs) at I/O time. This allows the ublk server to access the shared
per I/O — instead, it relies on the kernel matching physical pages
at I/O time. This allows the ublk server to access the shared
buffer directly, which is unlikely for the io_uring fixed buffer
approach.
@@ -507,8 +507,7 @@ tells the server where the data already lives.
``UBLK_F_SHMEM_ZC`` can be thought of as a supplement for optimized client
applications — when the client is willing to allocate I/O buffers from
shared memory, the entire data path becomes zero-copy without any per-I/O
overhead.
shared memory, the entire data path becomes zero-copy.
Use Cases
~~~~~~~~~
@@ -584,6 +583,9 @@ Limitations
the page cache, which allocates its own pages. These kernel-allocated
pages will never match the registered shared buffer. Only ``O_DIRECT``
puts the client's buffer pages directly into the block I/O.
- **Contiguous data only**: each I/O request's data must be contiguous
within a single registered buffer. Scatter/gather I/O that spans
multiple non-adjacent registered buffers cannot use the zero-copy path.
Control Commands
~~~~~~~~~~~~~~~~