[PATCH 6/7] Documentation: ublk: address review comments for SHMEM_ZC docs

Ming Lei
2026-04-09T13:30:55Z

- 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>
---
Documentation/block/ublk.rst | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst
index a818e09a4b66..c39d111af2d2 100644
--- a/Documentation/block/ublk.rst
+++ b/Documentation/block/ublk.rst
@@ -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
~~~~~~~~~~~~~~~~
--
2.53.0