[PATCH v2 05/24] man/man7/sock_diag.7: udiag_wqueue: This is memory in use, not memory available

["Günther Noack" <gnoack3000@gmail.com>]

The reported value is equivalent to what SIOCINQ/SIOCOUTQ report, as
also discussed in the commit message in linux.git c9da99e6475f
(2011-12-30; "unix_diag: Fixup RQLEN extension report").  (Those are
socket-flavoured synonyms of TIOCINQ(2const), TIOCOUTQ(2const))

In Linux code, the reasoning is as follows:

* The udiag_wqueue field is set in sk_diag_show_rqlen() in linux.git
  net/unix/diag.c, where for non-listening sockets, unix_outq_len() is
  assigned.
* unix_outq_len() in linux.git net/unix/af_unix.c returns
  sk_wmem_alloc_get().
* sk_wmem_alloc_get() in linux.git include/net/sock.h reads its return
  value from refcount_read(&sk->sk_wmem_alloc) (with some technical
  tweaks).
* The field sk_wmem_alloc is documented in the same file as:
  @sk_wmem_alloc: transmit queue bytes committed

That this interpretation of sk_wmem_alloc is correct can also be seen
in the places where the protocol-specific code adds to it.
The UNIX sockets increment sk->sk_wmem_alloc as follows:

* unix_dgram_sendmsg() and unix_stream_sendmsg() in net/unix/af_unix.c
  invoke sock_alloc_send_pskb()
* sock_alloc_send_pskb() in net/core/sock.c calls skb_set_owner_w()
* skb_set_owner_w() in the same file increments the counter using
  __refcount_add(skb->truesize, &sk->sk_wmem_alloc, &old_wmem);

Fixes: 4f6a0a4a909e (2016-12-07; "sock_diag.7: New page documenting NETLINK_SOCK_DIAG interface")
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Cc: Dmitry V. Levin <ldv@strace.io>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Assisted-by: LLM
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---

Notes:
    Verified in kernel code; the udiag_wqueue field does indeed have these
    different semantics -- it's the data already sent, which is still
    stuck in the output queue, not the maximum size of the output queue.

 man/man7/sock_diag.7 | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/man/man7/sock_diag.7 b/man/man7/sock_diag.7
index 35a6d5a80ffc..8f8b28b4404d 100644
--- a/man/man7/sock_diag.7
+++ b/man/man7/sock_diag.7
@@ -248,7 +248,9 @@ the backlog size which equals to the value passed as the second argument to
 .BR listen (2).
 .IP
 For established sockets:
-the amount of memory available for sending.
+the amount of memory currently allocated for data
+that was sent on this socket
+but has not yet been received by the peer.
 .RE
 .TP
 .B UDIAG_SHOW_MEMINFO
-- 
2.56.0