netdev - Re: [PATCH net] net/mlx5: Correctly set gso

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Message-ID: <CADg4-L-7UWVfWOAFOBjVJ4PXbz06b1riDO3r5d4QpGj+aTVcfw@mail.gmail.com>
Date: Wed, 30 Jul 2025 09:31:42 -0700
From: Christoph Paasch <cpaasch@...nai.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Gal Pressman <gal@...dia.com>, Willem de Bruijn <willemb@...gle.com>, Bailey Forrest <bcf@...gle.com>, 
	Catherine Sullivan <csully@...gle.com>, Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan <tariqt@...dia.com>, 
	Mark Bloch <mbloch@...dia.com>, Leon Romanovsky <leon@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Amir Vadai <amirv@...lanox.com>, netdev@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH net] net/mlx5: Correctly set gso_segs when LRO is used

On Wed, Jul 30, 2025 at 5:28 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Jul 30, 2025 at 4:06 AM Gal Pressman <gal@...dia.com> wrote:
> >
> > On 29/07/2025 21:34, Christoph Paasch via B4 Relay wrote:
> > > From: Christoph Paasch <cpaasch@...nai.com>
> > >
> > > When gso_segs is left at 0, a number of assumptions will end up being
> > > incorrect throughout the stack.
> > >
> > > For example, in the GRO-path, we set NAPI_GRO_CB()->count to gso_segs.
> > > So, if a non-LRO'ed packet followed by an LRO'ed packet is being
> > > processed in GRO, the first one will have NAPI_GRO_CB()->count set to 1 and
> > > the next one to 0 (in dev_gro_receive()).
> > > Since commit 531d0d32de3e
> > > ("net/mlx5: Correctly set gso_size when LRO is used")
> > > these packets will get merged (as their gso_size now matches).
> > > So, we end up in gro_complete() with NAPI_GRO_CB()->count == 1 and thus
> > > don't call inet_gro_complete(). Meaning, checksum-validation in
> > > tcp_checksum_complete() will fail with a "hw csum failure".
> > >
> > > Even before the above mentioned commit, incorrect gso_segs means that other
> > > things like TCP's accounting of incoming packets (tp->segs_in,
> > > data_segs_in, rcv_ooopack) will be incorrect. Which means that if one
> > > does bytes_received/data_segs_in, the result will be bigger than the
> > > MTU.
> > >
> > > Fix this by initializing gso_segs correctly when LRO is used.
> > >
> > > Fixes: e586b3b0baee ("net/mlx5: Ethernet Datapath files")
> >
> > Maybe we should put an additional Fixes line for the gso_size patch?
> > It doesn't directly fix it, but it will clearly emphasize the importance
> > of picking up this patch together with the other one.
> >
> > > Reported-by: Gal Pressman <gal@...dia.com>
> > > Closes: https://lore.kernel.org/netdev/6583783f-f0fb-4fb1-a415-feec8155bc69@nvidia.com/
> > > Signed-off-by: Christoph Paasch <cpaasch@...nai.com>
> >
> > Thanks Christoph,
> > Reviewed-by: Gal Pressman <gal@...dia.com>
>
> I do not think we need many Fixes: tag.
>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
>
> If we really want to be precise, the issue also came when GRO got
> support for GRO packets ;)
>
> commit 5eddb24901ee    gro: add support of (hw)gro packets to gro stack
>
> This commit really implied that both gso_size and gso_segs had to be
> set by drivers RX paths.
>
> It seems drivers/net/ethernet/google/gve/gve_rx_dqo.c has a similar issue.
>
> gve_rx_complete_rsc() sets gso_size but not gso_segs
>
> shinfo->gso_size = le16_to_cpu(desc->rsc_seg_len);

I see! I can send a fix, but won't have the ability to actually test
it. So, maybe better if someone else takes this one.

Christoph