Re: [PATCH net-next v3 0/2] Remove expired routes with a separated list of routes.

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2023-07-18 at 11:33 -0700, Kui-Feng Lee wrote:
> FIB6 GC walks trees of fib6_tables to remove expired routes. Walking a tree
> can be expensive if the number of routes in a table is big, even if most of
> them are permanent. Checking routes in a separated list of routes having
> expiration will avoid this potential issue.
> 
> Background
> ==========
> 
> The size of a Linux IPv6 routing table can become a big problem if not
> managed appropriately.  Now, Linux has a garbage collector to remove
> expired routes periodically.  However, this may lead to a situation in the routing path is blocked for a long period due to an
> excessive number of routes.
> 
> For example, years ago, there is a commit c7bb4b89033b ("ipv6: tcp: drop
> silly ICMPv6 packet too big messages") about "ICMPv6 Packet too big
> messages". The root cause is that malicious ICMPv6 packets were sent back
> for every small packet sent to them. These packets add routes with an
> expiration time that prompts the GC to periodically check all routes in the
> tables, including permanent ones.
> 
> Why Route Expires
> =================
> 
> Users can add IPv6 routes with an expiration time manually. However,
> the Neighbor Discovery protocol may also generate routes that can
> expire.  For example, Router Advertisement (RA) messages may create a
> default route with an expiration time. [RFC 4861] For IPv4, it is not
> possible to set an expiration time for a route, and there is no RA, so
> there is no need to worry about such issues.
> 
> Create Routes with Expires
> ==========================
> 
> You can create routes with expires with the  command.
> 
> For example,
> 
>     ip -6 route add 2001:b000:591::3 via fe80::5054:ff:fe12:3457 \ 
>         dev enp0s3 expires 30
> 
> The route that has been generated will be deleted automatically in 30
> seconds.
> 
> GC of FIB6
> ==========
> 
> The function called fib6_run_gc() is responsible for performing
> garbage collection (GC) for the Linux IPv6 stack. It checks for the
> expiration of every route by traversing the trees of routing
> tables. The time taken to traverse a routing table increases with its
> size. Holding the routing table lock during traversal is particularly
> undesirable. Therefore, it is preferable to keep the lock for the
> shortest possible duration.
> 
> Solution
> ========
> 
> The cause of the issue is keeping the routing table locked during the
> traversal of large trees. To solve this problem, we can create a separate
> list of routes that have expiration. This will prevent GC from checking
> permanent routes.
> 
> Result
> ======
> 
> We conducted a test to measure the execution times of fib6_gc_timer_cb()
> and observed that it enhances the GC of FIB6. During the test, we added
> permanent routes with the following numbers: 1000, 3000, 6000, and
> 9000. Additionally, we added a route with an expiration time.
> 
> Here are the average execution times for the kernel without the patch.
>  - 120020 ns with 1000 permanent routes
>  - 308920 ns with 3000 ...
>  - 581470 ns with 6000 ...
>  - 855310 ns with 9000 ...
> 
> The kernel with the patch consistently takes around 14000 ns to execute,
> regardless of the number of permanent routes that are installed.
> 
> Major changes from v2:
> 
>  - Remove unnecessary and incorrect sysctl restoring in the test case.
> 
> Major changes from v1:
> 
>  - Moved gc_link to avoid creating a hole in fib6_info.
> 
>  - Moved fib6_set_expires*() and fib6_clean_expires*() to the header
>    file and inlined. And removed duplicated lines.
> 
>  - Added a test case.
> 
> ---
> v1: https://lore.kernel.org/all/20230710203609.520720-1-kuifeng@meta.com/
> v2: https://lore.kernel.org/all/20230718180321.294721-1-kuifeng@meta.com/

Too bad I did not notice v3 before starting reviewing v2.

When posting a new version you must wait the 24h quarantine period,
see:

https://elixir.bootlin.com/linux/v6.4/source/Documentation/process/maintainer-netdev.rst#L15

I assume this does not cope with the feedback on previous version ;)

/P