netdev - Re: [PATCH net-next 05/10] udp: refine __udp_enqueue_schedule

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Message-ID: <CAAVpQUCumbFexO9TBhed0G0wGToLc4crVMSh8OxqwLep6kSzuA@mail.gmail.com>
Date: Wed, 17 Sep 2025 10:53:11 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	"David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Willem de Bruijn <willemb@...gle.com>, David Ahern <dsahern@...nel.org>, 
	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next 05/10] udp: refine __udp_enqueue_schedule_skb() test

On Wed, Sep 17, 2025 at 8:57 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Sep 17, 2025 at 8:00 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Eric Dumazet wrote:
> > > Commit 5a465a0da13e ("udp: Fix multiple wraparounds
> > > of sk->sk_rmem_alloc.") allowed to slightly overshoot
> > > sk->sk_rmem_alloc, when many cpus are trying
> > > to feed packets to a common UDP socket.
> > >
> > > This patch, combined with the following one reduces
> > > false sharing on the victim socket under DDOS.
> >
> > It also changes the behavior. There was likely a reason to allow
> > at least one packet if the buffer is small. Kuniyuki?
>
> It should not change the behavior.
>
> rmem would be zero if there is no packet in the queue : We still
> accept the incoming skb, regardless of its truesize.
>
> If there is any packet, rmem > 0

Agreed, this change should be fine.

The rule comes from 0fd7bac6b6157, and later 850cbaddb52d
tried to be more strict but caused regression, and the condition
was converted to the current form in 363dc73acacbb.

Reviewed-by: Kuniyuki Iwashima <kuniyu@...gle.com>