linux-kernel - Re: [PATCH net-next V2 08/11] net/mlx5e: Convert over to netmem

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Message-ID: <aDDLD_JoKJs5iv2q@x130>
Date: Fri, 23 May 2025 12:22:55 -0700
From: Saeed Mahameed <saeed@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Tariq Toukan <tariqt@...dia.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Saeed Mahameed <saeedm@...dia.com>,
	Leon Romanovsky <leon@...nel.org>,
	Richard Cochran <richardcochran@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org, Moshe Shemesh <moshe@...dia.com>,
	Mark Bloch <mbloch@...dia.com>, Gal Pressman <gal@...dia.com>,
	Cosmin Ratiu <cratiu@...dia.com>,
	Dragos Tatulea <dtatulea@...dia.com>
Subject: Re: [PATCH net-next V2 08/11] net/mlx5e: Convert over to netmem

On 23 May 10:58, Mina Almasry wrote:
>On Thu, May 22, 2025 at 4:54 PM Saeed Mahameed <saeed@...nel.org> wrote:
>> >>  static inline void
>> >>  mlx5e_copy_skb_header(struct mlx5e_rq *rq, struct sk_buff *skb,
>> >> -                     struct page *page, dma_addr_t addr,
>> >> +                     netmem_ref netmem, dma_addr_t addr,
>> >>                       int offset_from, int dma_offset, u32 headlen)
>> >>  {
>> >> -       const void *from = page_address(page) + offset_from;
>> >> +       const void *from = netmem_address(netmem) + offset_from;
>> >
>> >I think this needs a check that netmem_address != NULL and safe error
>> >handling in case it is? If the netmem is unreadable, netmem_address
>> >will return NULL, and because you add offset_from to it, you can't
>> >NULL check from as well.
>> >
>>
>> Nope, this code path is not for GRO_HW, it is always safe to assume this is
>> not iov_netmem.
>>
>
>OK, thanks for checking. It may be worth it to add
>DEBUG_NET_WARN_ON_ONCE(netmem_address(netmem)); in these places where

Too ugly and will only be caught in DEBUG env with netmem_iov enabled on a
somehow broken driver, so if you already doing that I am sure
you won't mind a crash :) in your debug env.. 

Also I don't expect any of mlx5 developers to confuse between header data
split paths and other paths.. but maybe a comment somewhere should cover
this gap.

>you're assuming the netmem is readable and has a valid address. It
>would be a very subtle bug later on if someone moves the code or
>something and suddenly you have unreadable netmem being funnelled
>through these code paths. But up to you.
>

Cosmin, let's add comments on the shampo skb functions and the relevant
lines of code, maybe it will help preventing future mistakes.

>-- 
>Thanks,
>Mina
>