netdev - Re: [PATCHv5 iproute2 net-next 2/2] lib/libnetlink: re malloc buff if size is not enough

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Date:   Tue, 12 Feb 2019 15:43:24 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Michal Kubecek <mkubecek@...e.cz>, Phil Sutter <phil@....cc>
Subject: Re: [PATCHv5 iproute2 net-next 2/2] lib/libnetlink: re malloc buff if
 size is not enough



On 02/12/2019 03:32 PM, Eric Dumazet wrote:
> 

> 
> This patch brings a serious performance penalty.
> 
> ss command now uses two system calls per ~4KB worth of data
> 
> recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{NULL, 0}], msg_controllen=0, msg_flags=MSG_TRUNC}, MSG_PEEK|MSG_TRUNC) = 3328 <0.000120>
> recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"h\0\0\0\24\0\2\0@\342\1\0\322\0\6\0\n\1\1\0\250\253\276@&\7\370\260\200\231\16\6"..., 3328}], msg_controllen=0, msg_flags=0}, 0) = 3328 <0.000108>
> recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{NULL, 0}], msg_controllen=0, msg_flags=MSG_TRUNC}, MSG_PEEK|MSG_TRUNC) = 3328 <0.000086>
> recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"h\0\0\0\24\0\2\0@\342\1\0\322\0\6\0\n\10\2\0002A\266S&\7\370\260\200\231\16\6"..., 3328}], msg_controllen=0, msg_flags=0}, 0) = 3328 <0.000121>
> 
> 
> So we are back to a very pessimistic situation.
> 

I guess this patch will solve the issue :

diff --git a/lib/libnetlink.c b/lib/libnetlink.c
index ced33728777a17e0905e76acb904ac4709707488..309b5b3787e3d8f8c47f035d270ae2b4df01703e 100644
--- a/lib/libnetlink.c
+++ b/lib/libnetlink.c
@@ -442,6 +442,8 @@ static int rtnl_recvmsg(int fd, struct msghdr *msg, char **answer)
        if (len < 0)
                return len;
 
+       if (len < 32768)
+               len = 32768;
        buf = malloc(len);
        if (!buf) {
                fprintf(stderr, "malloc error: not enough buffer\n");