netdev - Re: [PATCH net] ipv6: Align behavior across nexthops during path selection

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Message-ID: <f5f89b3a-c532-477d-9dcf-7362de336777@kernel.org>
Date: Tue, 8 Apr 2025 09:17:38 -0600
From: David Ahern <dsahern@...nel.org>
To: Ido Schimmel <idosch@...dia.com>, netdev@...r.kernel.org
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
 edumazet@...gle.com, horms@...nel.org, willemdebruijn.kernel@...il.com
Subject: Re: [PATCH net] ipv6: Align behavior across nexthops during path
 selection

On 4/8/25 2:43 AM, Ido Schimmel wrote:
> A nexthop is only chosen when the calculated multipath hash falls in the
> nexthop's hash region (i.e., the hash is smaller than the nexthop's hash
> threshold) and when the nexthop is assigned a non-negative score by
> rt6_score_route().
> 
> Commit 4d0ab3a6885e ("ipv6: Start path selection from the first
> nexthop") introduced an unintentional difference between the first
> nexthop and the rest when the score is negative.
> 
> When the first nexthop matches, but has a negative score, the code will
> currently evaluate subsequent nexthops until one is found with a
> non-negative score. On the other hand, when a different nexthop matches,
> but has a negative score, the code will fallback to the nexthop with
> which the selection started ('match').
> 
> Align the behavior across all nexthops and fallback to 'match' when the
> first nexthop matches, but has a negative score.
> 
> Fixes: 3d709f69a3e7 ("ipv6: Use hash-threshold instead of modulo-N")
> Fixes: 4d0ab3a6885e ("ipv6: Start path selection from the first nexthop")
> Reported-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Closes: https://lore.kernel.org/netdev/67efef607bc41_1ddca82948c@willemb.c.googlers.com.notmuch/
> Signed-off-by: Ido Schimmel <idosch@...dia.com>
> ---
>  net/ipv6/route.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 

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