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Message-Id: <20220323233715.2104106-2-kuba@kernel.org>
Date: Wed, 23 Mar 2022 16:37:13 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: davem@...emloft.net
Cc: netdev@...r.kernel.org, pabeni@...hat.com, corbet@....net,
imagedong@...cent.com, edumazet@...gle.com, dsahern@...nel.org,
talalahmad@...gle.com, linux-doc@...r.kernel.org,
Jakub Kicinski <kuba@...nel.org>
Subject: [RFC net-next 1/3] skbuff: add a basic intro doc
Add basic skb documentation. It's mostly an intro to the subsequent
patches - it would looks strange if we documented advanced topics
without covering the basics in any way.
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
---
Documentation/networking/skbuff.rst | 25 ++++++++++++++++++
include/linux/skbuff.h | 40 +++++++++++++++++++++++++++++
2 files changed, 65 insertions(+)
create mode 100644 Documentation/networking/skbuff.rst
diff --git a/Documentation/networking/skbuff.rst b/Documentation/networking/skbuff.rst
new file mode 100644
index 000000000000..7c6be64f486a
--- /dev/null
+++ b/Documentation/networking/skbuff.rst
@@ -0,0 +1,25 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+struct sk_buff
+==============
+
+:c:type:`struct sk_buff` is the main networking structure representing
+a packet.
+
+Basic sk_buff geometry
+----------------------
+
+.. kernel-doc:: include/linux/skbuff.h
+ :doc: Basic sk_buff geometry
+
+Shared skbs and skb clones
+--------------------------
+
+:c:member:`sk_buff.users` is a simple refcount allowing multiple entities
+to keep a struct sk_buff alive. skbs with a ``sk_buff.users != 1`` are referred
+to as shared skbs (see skb_shared()).
+
+skb_clone() allows for fast duplication of skbs. None of the data buffers
+get copied, but caller gets a new metadata struct (struct sk_buff).
+&skb_shared_info.refcount indicates the number of skbs pointing at the same
+packet data (i.e. clones).
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 3a30cae8b0a5..5431be4aa309 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -764,6 +764,46 @@ typedef unsigned int sk_buff_data_t;
typedef unsigned char *sk_buff_data_t;
#endif
+/**
+ * DOC: Basic sk_buff geometry
+ *
+ * struct sk_buff itself is a metadata structure and does not hold any packet
+ * data. All the data is held in associated buffers.
+ *
+ * &sk_buff.head points to the main "head" buffer. The head buffer is divided
+ * into two parts:
+ *
+ * - data buffer, containing headers and sometimes payload data;
+ * this is the part of the skb operated on by the common helpers
+ * such as skb_put() or skb_pull();
+ * - shared info (struct skb_shared_info) which holds an array of pointers
+ * to read-only payload data in the (page, offset, length) format.
+ *
+ * Optionally &skb_shared_info.frag_list may point to another skb.
+ *
+ * Basic diagram may look like this::
+ *
+ * ---------------
+ * | sk_buff |
+ * ---------------
+ * ,--------------------------- + head
+ * / ,----------------- + data
+ * / / ,----------- + tail
+ * | | | , + end
+ * | | | |
+ * v v v v
+ * -----------------------------------------------
+ * | headroom | data | tailroom | skb_shared_info |
+ * -----------------------------------------------
+ * + [page frag]
+ * + [page frag]
+ * + [page frag]
+ * + [page frag] ---------
+ * + frag_list --> | sk_buff |
+ * ---------
+ *
+ */
+
/**
* struct sk_buff - socket buffer
* @next: Next buffer in list
--
2.34.1
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