netdev - Re: [PATCH net-next 09/15] net: increase MAX_SKB

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Date:   Thu, 3 Feb 2022 11:54:45 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Coco Li <lixiaoyan@...gle.com>
Subject: Re: [PATCH net-next 09/15] net: increase MAX_SKB_FRAGS

On Thu, Feb 3, 2022 at 11:20 AM Eric Dumazet <edumazet@...gle.com> wrote:

>
> Another issue with CONFIG_ options is that they are integer.
>
> Trying the following did not work
>
> #define MAX_SKB_FRAGS  ((unsigned long)CONFIG_MAX_SKB_FRAGS)
>
> Because in some places we have
>
> #if    (   MAX_SKB_FRAGS > ...)
>
> (MAX_SKB_FRAGS is UL currently, making it an integer might cause some
> signed/unsigned operations buggy)

I came to something like this, clearly this a bit ugly.

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 08c12c41c5a5907dccc7389f396394d8132d962e..cc3cac3ee109f95c8a51eb90ba4a3bf7bebe86eb
100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -323,7 +323,15 @@ enum skb_drop_reason {
        SKB_DROP_REASON_MAX,
 };

+#ifdef CONFIG_MAX_SKB_FRAGS_17
+#define MAX_SKB_FRAGS 17UL
+#endif
+#ifdef CONFIG_MAX_SKB_FRAGS_25
+#define MAX_SKB_FRAGS 25UL
+#endif
+#ifdef CONFIG_MAX_SKB_FRAGS_45
 #define MAX_SKB_FRAGS 45UL
+#endif

 extern int sysctl_max_skb_frags;

diff --git a/net/Kconfig b/net/Kconfig
index 8a1f9d0287de3c32040eee03b60114c6e6d150bc..d91027a654c2aad7bfa55152ef81c882bf394aff
100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -253,6 +253,29 @@ config PCPU_DEV_REFCNT
          network device refcount are using per cpu variables if this
option is set.
          This can be forced to N to detect underflows (with a
performance drop).

+choice
+       prompt "Maximum number of fragments per skb_shared_info"
+       default MAX_SKB_FRAGS_17
+
+config MAX_SKB_FRAGS_17
+       bool "17 fragments per skb_shared_info"
+       help
+         Some drivers have assumptions about MAX_SKB_FRAGS being 17.
+         Until they are fixed, it is safe to adopt the old limit.
+
+config MAX_SKB_FRAGS_25
+       bool "25 fragments per skb_shared_info"
+       help
+         Helps BIG TCP workloads, but might expose bugs in some legacy drivers.
+
+config MAX_SKB_FRAGS_45
+       bool "45 fragments per skb_shared_info"
+       help
+         Helps BIG TCP workloads, but might expose bugs in some legacy drivers.
+         This also increase memory overhead of small packets.
+
+endchoice
+
 config RPS
        bool
        depends on SMP && SYSFS