[PATCH net-next v6 5/6] net: devmem: document SO_DEVMEM_AUTORELEASE socket option

Bobby Eshleman
2025-11-05T01:23:28Z

From: Bobby Eshleman <bobbyeshleman@meta.com>

Update devmem.rst documentation to describe the new SO_DEVMEM_AUTORELEASE
socket option and its usage.

Document the following:
- The two token release modes (automatic vs manual)
- How to use SO_DEVMEM_AUTORELEASE to control the behavior
- Performance benefits of disabling autorelease (~10% CPU reduction)
- Restrictions and caveats of manual token release
- Usage examples for both getsockopt and setsockopt

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
Documentation/networking/devmem.rst | 70 +++++++++++++++++++++++++++++++++++--
1 file changed, 68 insertions(+), 2 deletions(-)

diff --git a/Documentation/networking/devmem.rst b/Documentation/networking/devmem.rst
index a6cd7236bfbd..1bfce686dce6 100644
--- a/Documentation/networking/devmem.rst
+++ b/Documentation/networking/devmem.rst
@@ -215,8 +215,8 @@ Freeing frags
-------------

Frags received via SCM_DEVMEM_DMABUF are pinned by the kernel while the user
-processes the frag. The user must return the frag to the kernel via
-SO_DEVMEM_DONTNEED::
+processes the frag. Users should return tokens to the kernel via
+SO_DEVMEM_DONTNEED when they are done processing the data::

ret = setsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_DONTNEED, &token,
sizeof(token));
@@ -235,6 +235,72 @@ can be less than the tokens provided by the user in case of:
(a) an internal kernel leak bug.
(b) the user passed more than 1024 frags.

+
+Autorelease Control
+~~~~~~~~~~~~~~~~~~~
+
+The SO_DEVMEM_AUTORELEASE socket option controls what happens to outstanding
+tokens (tokens not released via SO_DEVMEM_DONTNEED) when the socket closes::
+
+ int autorelease = 0; /* 0 = manual release, 1 = automatic release */
+ ret = setsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_AUTORELEASE,
+ &autorelease, sizeof(autorelease));
+
+ /* Query current setting */
+ int current_val;
+ socklen_t len = sizeof(current_val);
+ ret = getsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_AUTORELEASE,
+ &current_val, &len);
+
+When autorelease is disabled (default):
+
+- Outstanding tokens are NOT released when the socket closes
+- Outstanding tokens are only released when the dmabuf is unbound
+- Provides better performance by eliminating xarray overhead (~10% CPU reduction)
+- Kernel tracks tokens via atomic reference counters in net_iov structures
+
+When autorelease is enabled:
+
+- Outstanding tokens are automatically released when the socket closes
+- Backwards compatible behavior
+- Kernel tracks tokens in an xarray per socket
+
+Important: In both modes, applications should call SO_DEVMEM_DONTNEED to
+return tokens as soon as they are done processing. The autorelease setting only
+affects what happens to tokens that are still outstanding when close() is called.
+
+The autorelease setting can only be changed when the socket has no outstanding
+tokens. If tokens are present, setsockopt returns -EBUSY.
+
+
+Performance Considerations
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Disabling autorelease provides approximately ~10% CPU utilization improvement in
+RX workloads by:
+
+- Eliminating xarray allocations and lookups for token tracking
+- Using atomic reference counters instead
+- Reducing lock contention on the xarray spinlock
+
+However, applications must ensure all tokens are released via
+SO_DEVMEM_DONTNEED before closing the socket, otherwise the backing pages will
+remain pinned until the dmabuf is unbound.
+
+
+Caveats
+~~~~~~~
+
+- With autorelease disabled, sockets cannot switch between different dmabuf
+ bindings. This restriction exists because tokens in this mode do not encode
+ the binding information necessary to perform the token release.
+
+- Applications using manual release mode (autorelease=0) must ensure all tokens
+ are returned via SO_DEVMEM_DONTNEED before socket close to avoid resource
+ leaks during the lifetime of the dmabuf binding. Tokens not released before
+ close() will only be freed when the dmabuf is unbound.
+
+
TX Interface
============

--
2.47.3