Re: [net-next] ipv6: sr: switch to GFP_ATOMIC flag to allocate memory during seg6local LWT setup

[David Ahern <dsahern@kernel.org>]

On 4/29/25 6:24 AM, Andrea Mayer wrote:
> Recent updates to the locking mechanism that protects IPv6 routing tables
> [1] have affected the SRv6 networking subsystem. Such changes cause
> problems with some SRv6 Endpoints behaviors, like End.B6.Encaps and also
> impact SRv6 counters.
> 
> Starting from commit 169fd62799e8 ("ipv6: Get rid of RTNL for SIOCADDRT and
> RTM_NEWROUTE."), the inet6_rtm_newroute() function no longer needs to
> acquire the RTNL lock for creating and configuring IPv6 routes and set up
> lwtunnels.
> The RTNL lock can be avoided because the ip6_route_add() function
> finishes setting up a new route in a section protected by RCU.
> This makes sure that no dev/nexthops can disappear during the operation.
> Because of this, the steps for setting up lwtunnels - i.e., calling
> lwtunnel_build_state() - are now done in a RCU lock section and not
> under the RTNL lock anymore.
> 
> However, creating and configuring a lwtunnel instance in an
> RCU-protected section can be problematic when that tunnel needs to
> allocate memory using the GFP_KERNEL flag.
> For example, the following trace shows what happens when an SRv6
> End.B6.Encaps behavior is instantiated after commit 169fd62799e8 ("ipv6:
> Get rid of RTNL for SIOCADDRT and RTM_NEWROUTE."):
> 

...

> 
> To solve this issue, we replace the GFP_KERNEL flag with the GFP_ATOMIC
> one in those SRv6 Endpoints that need to allocate memory during the
> setup phase. This change makes sure that memory allocations are handled
> in a way that works with RCU critical sections.
> 
> [1] - https://lore.kernel.org/all/20250418000443.43734-1-kuniyu@amazon.com/
> 
> Fixes: 169fd62799e8 ("ipv6: Get rid of RTNL for SIOCADDRT and RTM_NEWROUTE.")
> Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
> ---
>  net/ipv6/seg6_local.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>