[Devel] Re: [PATCH] [NETNS49] support for per/namespace routing cache cleanup

[Denis V. Lunev]

Daniel Lezcano wrote:
> Denis V. Lunev wrote:
>> Daniel Lezcano wrote:
>>> Denis V. Lunev wrote:
>>>> /proc/sys/net/route/flush should be accessible inside the net 
>>>> namespace.
>>>> Though, the complete opening of this file will result in a DoS or
>>>> significant entire host slowdown if a namespace process will 
>>>> continually
>>>> flush routes.
>>>>
>>>> This patch introduces per/namespace route flush facility.
>>>>
>>>> Each namespace wanted to flush a cache copies global generation 
>>>> count to
>>>> itself and starts the timer. The cache is dropped for a specific 
>>>> namespace
>>>> iff the namespace counter is greater or equal global ones.
>>>>
>>>> So, in general, unwanted namespaces do nothing. They hold very old low
>>>> counter and they are unaffected by the requested cleanup.
>>>>
>>>> Signed-of-by: Denis V. Lunev <den at openvz.org>
>>>
>>> That's right and that will happen when manipulating ip addresses of 
>>> the network devices too. But I am not confortable with your patchset. 
>>> It touches the routing flush function too hardly and it uses 
>>> current->nsproxy->net_ns.
>>>
>>> IMHO we should have two flush functions. One taking a network 
>>> namespace parameter and one without the network namespace parameter. 
>>> The first one is called when a write to 
>>> /proc/sys/net/ipv4/route/flush is done (we must use the network 
>>> namespace of the writer) or when a interface address is changed or 
>>> shutdown|up. The last one is called by the timer, so we have a global 
>>> timer flushing the routing cache for all the namespaces.
>>
>> we can't :( The unfortunate thing is that the actual cleanup is called 
>> indirectly and asynchronously. The user _schedule_ the garbage 
>> collector to run NOW and we are moving over a large routing cache. 
>> Really large.
>>
>> The idea to iterate over the list of each namespace to flush is bad. 
>> We are in atomic context. The list is protected by the mutex.
>>
>> The idea of several timers (per namespace) is also bad. You will 
>> iterate over large cache several times.
>>
>> No other acceptable way here for me :(.
>>
>> As for "the trigger" - rt_cache_flush, looks like you are right. We 
>> should pass namespace as a parameter. This should be done as a 
>> separate patch.
> 
> If we change:
> 
>     rt_cache_flush(struct net *net, int delay);
> 
> and inside we call rt_cache_flush((unsigned long)net);
> 
> And then we check in the rt_run_flush function,
> 
>     struct net *net = (struct net *)dummy;
> 
>     ...
>     for (i = rt_hash_mask; i >= 0; i--) {
>         ...
>         if (dummy && rth->fl.fl_net != net)
>             continue
>         ...
>     ...
> 
> So when rt_run_flush is called synchronously, the netns is specified in 
> dummy and only the routes belonging to netns are flushed. Otherwise when 
> it is called by the timer, netns is not set so all routes are flushed.
> 

this does not look good for me. The size of this cache for 4GB host is 
2*10^6 entries for IPv4 with a 131072 chains. The conventional 
mainstream kernel wants to merge the routing cache cleanup requests from 
the different sources if they are delayed (default).

The main idea for this patch is to protect all other namespaces from the 
current one. This cache is an important resource. You proposal will work 
for the forced synchronous cleanups. Though, there are some requests 
with results in the delayed rt_run_flush via [mod/add]_timer

How should we handle them?

Regards,
	Den