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Diffstat (limited to 'Documentation/block')
| -rw-r--r-- | Documentation/block/ublk.rst | 10 |
1 files changed, 6 insertions, 4 deletions
diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst index e80cc415a739..0413dcd9ef69 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 ~~~~~~~~~~~~~~~~ |