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

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Date: Sun, 4 Jun 2023 16:33:00 +0300
From: Gal Pressman <gal@...dia.com>
To: Stephen Hemminger <stephen@...workplumber.org>
Cc: David Ahern <dsahern@...il.com>, Eric Dumazet <edumazet@...gle.com>,
 netdev <netdev@...r.kernel.org>, Eric Dumazet <eric.dumazet@...il.com>,
 Hangbin Liu <liuhangbin@...il.com>, Phil Sutter <phil@....cc>,
 Jakub Kicinski <kuba@...nel.org>, Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [PATCH iproute2] lib/libnetlink: ensure a minimum of 32KB for the
 buffer used in rtnl_recvmsg()

On 01/06/2023 0:51, Stephen Hemminger wrote:
> On Mon, 29 May 2023 15:29:51 +0300
> Gal Pressman <gal@...dia.com> wrote:
> 
>> On 13/02/2019 4:04, 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.
>>>
>>> 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.
>>>   
>>
>> Hey,
>>
>> Sorry for reviving this old thread, but I see this topic was already
>> discussed here :).
>> I have a system where the large number of VFs result in a message
>> greater than 32k, which makes a simple 'ip link' command return an error.
>>
>> Should we change the kernel's 'max_recvmsg_len' to 64k? Any other (more
>> robust) ideas to resolve this issue?
> 
> No matter what the size, someone will always have too many VF's to fit
> in the response. There is no way to get a stable solution without doing
> some API changes.
> 
> It is possible to dump millions of routes, so it is not directly a netlink
> issue more that the current API is slamming all the VF's as info blocks
> under a single message response.
> 
> That would mean replacing IFLA_VFINFO_LIST with a separate query

Thanks Stephen!
How would you imagine it? Changing the userspace to split each (PF, VF)
to a separate netlink call instead of a single call for each PF?