diff options
| author | Eric Dumazet <edumazet@google.com> | 2023-03-23 09:28:42 -0700 |
|---|---|---|
| committer | Jakub Kicinski <kuba@kernel.org> | 2023-03-27 19:29:22 -0700 |
| commit | 3948b05950fdd64002a5f182c65ba5cf2d53cf71 (patch) | |
| tree | 6397f0b034631ef13231dc76df8352292dbd54b5 /include/linux/skbuff.h | |
| parent | e5b42483ccce50d5b957f474fd332afd4ef0c27b (diff) | |
net: introduce a config option to tweak MAX_SKB_FRAGS
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 add a config option
to be able to fit up to 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 MAX_SKB_FRAGS=45 value for years at Google before
we deployed 4K MTU, with no adverse effect, other than
a recent issue in mlx4, fixed in commit 26782aad00cc
("net/mlx4: MLX4_TX_BOUNCE_BUFFER_SIZE depends on MAX_SKB_FRAGS")
Back then, goal was to be able to receive full size (64KB) GRO
packets without the frag_list overhead.
Note that /proc/sys/net/core/max_skb_frags can also be used to limit
the number of fragments TCP can use in tx packets.
By default we keep the old/legacy value of 17 until we get
more coverage for the updated values.
Sizes of struct skb_shared_info on 64bit arches
MAX_SKB_FRAGS | sizeof(struct skb_shared_info):
==============================================
17 320
21 320+64 = 384
25 320+128 = 448
29 320+192 = 512
33 320+256 = 576
37 320+320 = 640
41 320+384 = 704
45 320+448 = 768
This inflation might cause problems for drivers assuming they could pack
both the incoming packet (for MTU=1500) and skb_shared_info in half a page,
using build_skb().
v3: fix build error when CONFIG_NET=n
v2: fix two build errors assuming MAX_SKB_FRAGS was "unsigned long"
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://lore.kernel.org/r/20230323162842.1935061-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Diffstat (limited to 'include/linux/skbuff.h')
| -rw-r--r-- | include/linux/skbuff.h | 16 |
1 files changed, 5 insertions, 11 deletions
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index fe661011644b..82511b2f61ea 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -345,18 +345,12 @@ struct sk_buff_head { struct sk_buff; -/* 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) +#ifndef CONFIG_MAX_SKB_FRAGS +# define CONFIG_MAX_SKB_FRAGS 17 #endif + +#define MAX_SKB_FRAGS CONFIG_MAX_SKB_FRAGS + extern int sysctl_max_skb_frags; /* Set skb_shinfo(skb)->gso_size to this in case you want skb_segment to |