netdev - Re: [PATCH net-next] ppp: enable TX scatter-gather

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Message-ID: <20250915181015.67588ec2@kernel.org>
Date: Mon, 15 Sep 2025 18:10:15 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Qingfang Deng <dqfext@...il.com>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, linux-ppp@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Felix Fietkau <nbd@....name>
Subject: Re: [PATCH net-next] ppp: enable TX scatter-gather

On Fri, 12 Sep 2025 17:59:27 +0800 Qingfang Deng wrote:
> When chan->direct_xmit is true, and no compressors are in use, PPP
> prepends its header to a skb, and calls dev_queue_xmit directly. In this
> mode the skb does not need to be linearized.
> Enable NETIF_F_SG and NETIF_F_FRAGLIST, and add .ndo_fix_features()
> callback to conditionally disable them if a linear skb is required.
> This is required to support PPPoE GSO.

Seems a bit racy. We can't netdev_update_features() under the spin lock
so there's going to be a window of time where datapath will see new
state but netdev flags won't be cleared, yet?

We either need to add a explicit linearization check in the xmit path,
or always reset the flags to disabled before we start tweaking the
config and re-enable after config (tho the latter feels like a bit of 
a hack).
-- 
pw-bot: cr