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.

Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 65 insertions, 0 deletions

diff --git a/Documentation/networking/skbuff.rst b/Documentation/networking/skbuff.rst
new file mode 100644
index 000000000000..b4a008feceb4
--- /dev/null
+++ b/Documentation/networking/skbuff.rst
@@ -0,0 +1,25 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+struct sk_buff
+==============
+
+:c:type:`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 97de40bcfef6..69802624276d 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -801,6 +801,46 @@ 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;
+ *    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 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
  *	@prev: Previous buffer in list