netdev - Re: skb_attempt_defer_free and reference counting

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Message-ID: <6C80F51E-F1AA-4FC8-B278-C73CAE2AA1F4@akamai.com>
Date: Fri, 31 Oct 2025 14:02:14 +0000
From: "Hudson, Nick" <nhudson@...mai.com>
To: Eric Dumazet <edumazet@...gle.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: skb_attempt_defer_free and reference counting



> On 31 Oct 2025, at 11:43, Eric Dumazet <edumazet@...gle.com> wrote:
> 
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> 
> On Fri, Oct 31, 2025 at 4:04 AM Hudson, Nick <nhudson@...mai.com> wrote:
>> 
>> Hi,
>> 
>> I’ve been looking at using skb_attempt_defer_free and had a question about the skb reference counting.
>> 
>> The existing reference release for any skb handed to skb_attempt_defer_free is done in skb_defer_free_flush (via napi_consume_skb). However, it seems to me that calling skb_attempt_defer_free on the same skb to drop the multiple references is problematic as, if the defer_list isn’t serviced between the calls, the list gets corrupted. That is, the skb can’t appear on the list twice.
>> 
>> Would it be possible to move the reference count drop into skb_attempt_defer_free and only add the skb to the list on last reference drop?
> 
> We do not plan using this helper for arbitrary skbs, but ones fully
> owned by TCP and UDP receive paths.

Interesting. 

This patch has shown to give a performance benefit and I’m curious if it problematic in any way.

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index fae1a0ab36bd..59ffac9afdad 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2251,7 +2251,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
                if (unlikely(ret < 0))
                        kfree_skb(skb);
                else
-                       consume_skb(skb);
+                       skb_attempt_defer_free(skb);
        }

        return ret;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f220306731da..525b2a2698c6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -7167,6 +7167,7 @@ nodefer:  kfree_skb_napi_cache(skb);
        if (unlikely(kick))
                kick_defer_list_purge(sd, cpu);
 }
+EXPORT_SYMBOL(skb_attempt_defer_free);

 static void skb_splice_csum_page(struct sk_buff *skb, struct page *page,
  


> 
> skb_share_check() must have been called before reaching them.
> 
> In any case using skb->next could be problematic with shared skb.

OK, so the assumption is skb->users is already 1. Perhaps there is an optimisation in skb_defer_free_flush if that is the case?



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