netdev - Re: [PATCH iproute2] lib/libnetlink: ensure a minimum of 32KB for the buffer used in rtnl

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Date:   Tue, 12 Feb 2019 19:04:17 -0700
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Hangbin Liu <liuhangbin@...il.com>, Phil Sutter <phil@....cc>
Subject: Re: [PATCH iproute2] lib/libnetlink: ensure a minimum of 32KB for the
 buffer used in rtnl_recvmsg()

On 2/12/19 6:58 PM, Eric Dumazet wrote:
> In the past, we tried to increase the buffer size up to 32 KB in order
> to reduce number of syscalls per dump.
> 
> Commit 2d34851cd341 ("lib/libnetlink: re malloc buff if size is not enough")
> brought the size back to 4KB because the kernel can not know the application
> is ready to receive bigger requests.
> 
> See kernel commits 9063e21fb026 ("netlink: autosize skb lengthes") and
> d35c99ff77ec ("netlink: do not enter direct reclaim from netlink_dump()")
> for more details.
> 
> Fixes: 2d34851cd341 ("lib/libnetlink: re malloc buff if size is not enough")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Hangbin Liu <liuhangbin@...il.com>
> Cc: Phil Sutter <phil@....cc>
> ---
>  lib/libnetlink.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/lib/libnetlink.c b/lib/libnetlink.c
> index 1892a02ab5d0d73776c9882ffc77edcd2c663d01..0d48a3d43cf03065dacbd419578ab10af56431a4 100644
> --- a/lib/libnetlink.c
> +++ b/lib/libnetlink.c
> @@ -718,6 +718,8 @@ 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");
> 

I believe that negates the whole point of 2d34851cd341 - which I have no
problem with. 2 recvmsg calls per message is overkill.

Do we know of any single message sizes > 32k? 2d34851cd341 cites
increasing VF's but at some point there is a limit. If not, the whole
PEEK thing should go away and we just malloc 32k (or 64k) buffers for
each recvmsg.