netdev - Re: route/max_size sysctl in ipv4

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Date:	Tue, 6 Jan 2015 09:11:08 -0800
From:	Ani Sinha <ani@...sta.com>
To:	Pádraig Brady <P@...igbrady.com>
Cc:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: route/max_size sysctl in ipv4

On Tue, Jan 6, 2015 at 4:41 AM, Pádraig Brady <P@...igbrady.com> wrote:
> On 06/01/15 00:56, Ani Sinha wrote:
>> On Mon, Jan 5, 2015 at 4:51 PM, David Miller <davem@...emloft.net> wrote:
>>> From: Ani Sinha <ani@...sta.com>
>>> Date: Mon, 5 Jan 2015 16:43:30 -0800
>>>
>>>> On Mon, Jan 5, 2015 at 4:36 PM, David Miller <davem@...emloft.net> wrote:
>>>>> From: Ani Sinha <ani@...sta.com>
>>>>> Date: Mon, 5 Jan 2015 15:48:11 -0800
>>>>>
>>>>>> I am looking at the code and it looks like since the route cache for
>>>>>> ipv4 was removed from the kernel, this sysctl parameter no longer
>>>>>> serves the same purpose. It does not look like it is even used in the
>>>>>> ipv4/route.c module. Is there an equivalent sysctl parameter limiting
>>>>>> the number of route entries in the kernel? Or is there now no
>>>>>> mechanism to limit the number of route entries?
>>>>>
>>>>> There is nothing to limit, since the cache was removed.
>>>>
>>>> Shouldn't the documentation be updated to reflect that? Also what's
>>>> the point of having a dummy variable that does nothing? Should we not
>>>> simply remove it?
>>>
>>> There is nothing to update, the behavior is completely transparent.
>>> Absolutely no cache entries exist, therefore the limit cannot be
>>> reached.
>>
>> I disagree. You are advertising a feature in an official documentation
>> that simply does not exist for ipv4. This is very confusing. If I did
>> not dig into the code, I wouldn't know that this particular knob is a
>> noop since the time the route cache was removed.
>
> You can't change APIs with impunity.
>
>>> The sysctl is kept so that scripts reading it don't suddenly stop
>>> working.  We can't just remove sysctl values.
>
> Perhaps /proc/sys/net/ipv4/route/max_size should always return 0 when read,
> and discard written values?

Why can't se simply change the documentation to reflect the fact that
this sysctl is no longer in operation?
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