2018-08-06 11:50:47

by Yafang Shao

Subject: [PATCH net-next] tcp: fix the calculation of sysctl_max_tw_buckets in tcp_sk_init()

tcp_hashinfo.ehash_mask is always an odd number, which is set in function
alloc_large_system_hash(). See bellow,
if (_hash_mask)
*_hash_mask = (1 << log2qty) - 1; <<< always odd number

Hence the local variable 'cnt' is a even number, as a result of that it is
no difference to do the incrementation here.

Maybe the compiler could also optimize it, but this code is a little ugly.

Fix: fee83d09 ("ipv4: Namespaceify tcp_max_syn_backlog knob")
Signed-off-by: Yafang Shao <[email protected]>
---
net/ipv4/tcp_ipv4.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 9e041fa..a9b7c4b 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2543,7 +2543,7 @@ static int __net_init tcp_sk_init(struct net *net)
net->ipv4.sysctl_tcp_tw_reuse = 2;

cnt = tcp_hashinfo.ehash_mask + 1;
- net->ipv4.tcp_death_row.sysctl_max_tw_buckets = (cnt + 1) / 2;
+ net->ipv4.tcp_death_row.sysctl_max_tw_buckets = cnt / 2;
net->ipv4.tcp_death_row.hashinfo = &tcp_hashinfo;

net->ipv4.sysctl_max_syn_backlog = max(128, cnt / 256);
--
1.8.3.1

2018-08-06 20:52:50

by David Miller

Subject: Re: [PATCH net-next] tcp: fix the calculation of sysctl_max_tw_buckets in tcp_sk_init()

From: Yafang Shao <[email protected]>
Date: Mon, 6 Aug 2018 19:47:00 +0800

> tcp_hashinfo.ehash_mask is always an odd number, which is set in function
> alloc_large_system_hash(). See bellow,
> if (_hash_mask)
> *_hash_mask = (1 << log2qty) - 1; <<< always odd number
>
> Hence the local variable 'cnt' is a even number, as a result of that it is
> no difference to do the incrementation here.
>
> Maybe the compiler could also optimize it, but this code is a little ugly.
>
> Fix: fee83d09 ("ipv4: Namespaceify tcp_max_syn_backlog knob")

The correct tag is "Fixes: "

> @@ -2543,7 +2543,7 @@ static int __net_init tcp_sk_init(struct net *net)
> net->ipv4.sysctl_tcp_tw_reuse = 2;
>
> cnt = tcp_hashinfo.ehash_mask + 1;
> - net->ipv4.tcp_death_row.sysctl_max_tw_buckets = (cnt + 1) / 2;
> + net->ipv4.tcp_death_row.sysctl_max_tw_buckets = cnt / 2;
> net->ipv4.tcp_death_row.hashinfo = &tcp_hashinfo;

This is completely harmless, and does no harm.

You aren't "fixing" anything.

2018-08-07 01:48:04

by Yafang Shao

Subject: Re: [PATCH net-next] tcp: fix the calculation of sysctl_max_tw_buckets in tcp_sk_init()

On Tue, Aug 7, 2018 at 4:41 AM, David Miller <[email protected]> wrote:
> From: Yafang Shao <[email protected]>
> Date: Mon, 6 Aug 2018 19:47:00 +0800
>
>> tcp_hashinfo.ehash_mask is always an odd number, which is set in function
>> alloc_large_system_hash(). See bellow,
>> if (_hash_mask)
>> *_hash_mask = (1 << log2qty) - 1; <<< always odd number
>>
>> Hence the local variable 'cnt' is a even number, as a result of that it is
>> no difference to do the incrementation here.
>>
>> Maybe the compiler could also optimize it, but this code is a little ugly.
>>
>> Fix: fee83d09 ("ipv4: Namespaceify tcp_max_syn_backlog knob")
>
> The correct tag is "Fixes: "

OK

>
>> @@ -2543,7 +2543,7 @@ static int __net_init tcp_sk_init(struct net *net)
>> net->ipv4.sysctl_tcp_tw_reuse = 2;
>>
>> cnt = tcp_hashinfo.ehash_mask + 1;
>> - net->ipv4.tcp_death_row.sysctl_max_tw_buckets = (cnt + 1) / 2;
>> + net->ipv4.tcp_death_row.sysctl_max_tw_buckets = cnt / 2;
>> net->ipv4.tcp_death_row.hashinfo = &tcp_hashinfo;
>
> This is completely harmless, and does no harm.
>

It is harmless, meanwhile, it is useless and a little ugly.

By the way, why do we init sysctl_max_tw_buckets to
(tcp_hashinfo.ehash_mask + 1) / 2 or (tcp_hashinfo.ehash_mask + 2) / 2
?
Per my pespective, this default value is a little huge, sometimes we
have to set this value to a lower value with sysctl, otherwise it may
easily cause tcp port exhaustion issue.
So why not just init it to a lower value ?

> You aren't "fixing" anything.