netdev - Re: [BUG] mlx5_core memory management issue

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Message-ID: <72vpwjc4tosqt2djhyatkycofi2hlktulevzlszmhb6w3mlo46@63sxu3or7suc>
Date: Tue, 12 Aug 2025 20:19:30 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Jesse Brandeburg <jbrandeburg@...udflare.com>, 
	Chris Arges <carges@...udflare.com>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, 
	kernel-team <kernel-team@...udflare.com>, Jesper Dangaard Brouer <hawk@...nel.org>, tariqt@...dia.com, 
	saeedm@...dia.com, Leon Romanovsky <leon@...nel.org>, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Simon Horman <horms@...nel.org>, Andrew Rzeznik <arzeznik@...udflare.com>, 
	Yan Zhai <yan@...udflare.com>
Subject: Re: [BUG] mlx5_core memory management issue

On Tue, Aug 12, 2025 at 11:55:39AM -0700, Jesse Brandeburg wrote:
> On 8/12/25 8:44 AM, 'Dragos Tatulea' via kernel-team wrote:
> 
> > diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> > index 482d284a1553..484216c7454d 100644
> > --- a/kernel/bpf/devmap.c
> > +++ b/kernel/bpf/devmap.c
> > @@ -408,8 +408,10 @@ static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
> >          /* If not all frames have been transmitted, it is our
> >           * responsibility to free them
> >           */
> > +       xdp_set_return_frame_no_direct();
> >          for (i = sent; unlikely(i < to_send); i++)
> >                  xdp_return_frame_rx_napi(bq->q[i]);
> > +       xdp_clear_return_frame_no_direct();
> 
> Why can't this instead just be xdp_return_frame(bq->q[i]); with no
> "no_direct" fussing?
> 
> Wouldn't this be the safest way for this function to call frame completion?
> It seems like presuming the calling context is napi is wrong?
>
It would be better indeed. Thanks for removing my horse glasses!

Once Chris verifies that this works for him I can prepare a fix patch.

> The other option here seems to be using the xdp_return_frame_bulk() but
> you'd need to be careful to make sure the rcu lock was taken or already
> held, but it should already be, since it's taken inside xdp_do_flush.
>
That would be even better, but bq_xmit_all() is also called by
bq_enqueue() which doesn't seem to have the rcu lock taken.

Thannks,
Dragos