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 18:08:13 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Ahern <dsahern@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev <netdev@...r.kernel.org>,
        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 02/12/2019 06:04 PM, David Ahern wrote:
> 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.
> 

It does not negates the point at all.

The main point was to eventually be able to allocate more than 32KB.

We need to have a minimum size of 32KB so that the kernel can cook reasonably sized skbs

Because trying to allocate 4KB only in 2019 is kind of stupid...

( Especially considering ss currently buffers the whole thing before calling render() !!! )

> 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.
>