Re: [RFC net-next 2/3] skbuff: rewrite the doc for data-only skbs
From: Paolo Abeni <pabeni@redhat.com>
Date: 2022-03-24 08:50:14
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linux-doc
Hello, On Wed, 2022-03-23 at 16:37 -0700, Jakub Kicinski wrote:
quoted hunk ↗ jump to hunk
The comment about shinfo->dataref split is really unhelpful, at least to me. Rewrite it and render it to skb documentation. Signed-off-by: Jakub Kicinski <kuba@kernel.org> --- Documentation/networking/index.rst | 1 + Documentation/networking/skbuff.rst | 6 ++++++ include/linux/skbuff.h | 33 +++++++++++++++++++---------- 3 files changed, 29 insertions(+), 11 deletions(-)diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst index ce017136ab05..1b3c45add20d 100644 --- a/Documentation/networking/index.rst +++ b/Documentation/networking/index.rst@@ -96,6 +96,7 @@ Linux Networking Documentation sctp secid seg6-sysctl + skbuff smc-sysctl statistics strparserdiff --git a/Documentation/networking/skbuff.rst b/Documentation/networking/skbuff.rst index 7c6be64f486a..581e5561c362 100644 --- a/Documentation/networking/skbuff.rst +++ b/Documentation/networking/skbuff.rst@@ -23,3 +23,9 @@ 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). + +dataref and headerless skbs +--------------------------- + +.. kernel-doc:: include/linux/skbuff.h + :doc: dataref and headerless skbsdiff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 5431be4aa309..5b838350931c 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h@@ -691,16 +691,25 @@ struct skb_shared_info { skb_frag_t frags[MAX_SKB_FRAGS]; }; -/* We divide dataref into two halves. The higher 16 bits hold references - * to the payload part of skb->data. The lower 16 bits hold references to - * the entire skb->data. A clone of a headerless skb holds the length of - * the header in skb->hdr_len. +/** + * DOC: dataref and headerless skbs + * + * Transport layers send out clones of data skbs they hold for retransmissions. + * To allow lower layers of the stack to prepend their headers + * we split &skb_shared_info.dataref into two halves. + * The lower 16 bits count the overall number of references. + * The higher 16 bits indicate number of data-only references. + * skb_header_cloned() checks if skb is allowed to add / write the headers.
Thank you very much for the IMHO much needed documentation! Please allow me to do some non-native-english-speaker biased comments;) The previous patch uses the form "payload data" instead of data-only, I think it would be clearer using consistently one or the other. I personally would go for "payload-data-only" (which is probably a good reason to pick a different option).
* - * All users must obey the rule that the skb->data reference count must be - * greater than or equal to the payload reference count. + * The creator of the skb (e.g. TCP) marks its data-only skb as &sk_buff.nohdr + * (via __skb_header_release()). Any clone created from marked skb will get + * &sk_buff.hdr_len populated with the available headroom. + * If it's the only clone in existence it's able to modify the headroom at will.
I think it would be great if we explicitly list the expected sequence, e.g. <alloc skb> skb_reserve __skb_header_release skb_clone Thanks! Paolo