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

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Message-Id: <20220203015140.3022854-10-eric.dumazet@gmail.com>
Date:   Wed,  2 Feb 2022 17:51:34 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Coco Li <lixiaoyan@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next 09/15] net: increase MAX_SKB_FRAGS

From: Eric Dumazet <edumazet@...gle.com>

Currently, MAX_SKB_FRAGS value is 17.

For standard tcp sendmsg() traffic, no big deal because tcp_sendmsg()
attempts order-3 allocations, stuffing 32768 bytes per frag.

But with zero copy, we use order-0 pages.

For BIG TCP to show its full potential, we increase MAX_SKB_FRAGS
to be able to fit 45 segments per skb.

This is also needed for BIG TCP rx zerocopy, as zerocopy currently
does not support skbs with frag list.

We have used this MAX_SKB_FRAGS value for years at Google before
we deployed 4K MTU, with no adverse effect.
Back then, goal was to be able to receive full size (64KB) GRO
packets without the frag_list overhead.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/linux/skbuff.h | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index a27bcc4f7e9a92ea4f4f4f6e5f454bb4f8099f66..08c12c41c5a5907dccc7389f396394d8132d962e 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -323,18 +323,8 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_MAX,
 };
 
-/* To allow 64K frame to be packed as single skb without frag_list we
- * require 64K/PAGE_SIZE pages plus 1 additional page to allow for
- * buffers which do not start on a page boundary.
- *
- * Since GRO uses frags we allocate at least 16 regardless of page
- * size.
- */
-#if (65536/PAGE_SIZE + 1) < 16
-#define MAX_SKB_FRAGS 16UL
-#else
-#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)
-#endif
+#define MAX_SKB_FRAGS 45UL
+
 extern int sysctl_max_skb_frags;
 
 /* Set skb_shinfo(skb)->gso_size to this in case you want skb_segment to
-- 
2.35.0.rc2.247.g8bbb082509-goog