netdev - RE: [PATCH net-next] net: mana: Add support for jumbo frame

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Message-ID: <PH7PR21MB31162F5F9E5C8C146760AF10CA809@PH7PR21MB3116.namprd21.prod.outlook.com>
Date:   Mon, 20 Mar 2023 01:47:18 +0000
From:   Haiyang Zhang <haiyangz@...rosoft.com>
To:     Francois Romieu <romieu@...zoreil.com>
CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Paul Rosswurm <paulros@...rosoft.com>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "leon@...nel.org" <leon@...nel.org>,
        Long Li <longli@...rosoft.com>,
        "ssengar@...ux.microsoft.com" <ssengar@...ux.microsoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net-next] net: mana: Add support for jumbo frame



> -----Original Message-----
> From: Francois Romieu <romieu@...zoreil.com>
> Sent: Sunday, March 19, 2023 6:47 PM
> To: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: linux-hyperv@...r.kernel.org; netdev@...r.kernel.org; Dexuan Cui
> <decui@...rosoft.com>; KY Srinivasan <kys@...rosoft.com>; Paul Rosswurm
> <paulros@...rosoft.com>; olaf@...fle.de; vkuznets@...hat.com;
> davem@...emloft.net; wei.liu@...nel.org; edumazet@...gle.com;
> kuba@...nel.org; pabeni@...hat.com; leon@...nel.org; Long Li
> <longli@...rosoft.com>; ssengar@...ux.microsoft.com; linux-
> kernel@...r.kernel.org
> Subject: Re: [PATCH net-next] net: mana: Add support for jumbo frame
> 
> [Some people who received this message don't often get email from
> romieu@...zoreil.com. Learn why this is important at
> https://aka.ms/LearnAboutSenderIdentification ]
> 
> Haiyang Zhang <haiyangz@...rosoft.com> :
> [...]
> > diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c
> b/drivers/net/ethernet/microsoft/mana/mana_en.c
> > index 492474b4d8aa..07738b7e85f2 100644
> > --- a/drivers/net/ethernet/microsoft/mana/mana_en.c
> > +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
> > @@ -427,6 +427,34 @@ static u16 mana_select_queue(struct net_device
> *ndev, struct sk_buff *skb,
> >       return txq;
> >  }
> >
> > +static int mana_change_mtu(struct net_device *ndev, int new_mtu)
> > +{
> > +     unsigned int old_mtu = ndev->mtu;
> > +     int err, err2;
> > +
> > +     err = mana_detach(ndev, false);
> > +     if (err) {
> > +             netdev_err(ndev, "mana_detach failed: %d\n", err);
> > +             return err;
> > +     }
> > +
> > +     ndev->mtu = new_mtu;
> > +
> > +     err = mana_attach(ndev);
> > +     if (!err)
> > +             return 0;
> > +
> > +     netdev_err(ndev, "mana_attach failed: %d\n", err);
> > +
> > +     /* Try to roll it back to the old configuration. */
> > +     ndev->mtu = old_mtu;
> > +     err2 = mana_attach(ndev);
> > +     if (err2)
> > +             netdev_err(ndev, "mana re-attach failed: %d\n", err2);
> > +
> > +     return err;
> > +}
> 
> I do not see where the driver could depend on the MTU. Even if it fails,
> a single call to mana_change_mtu should thus never wreck the old working
> state/configuration.
> 
> Stated differently, the detach/attach implementation is simple but
> it makes the driver less reliable than it could be.
> 
> No ?

No, it doesn't make the driver less reliable. To safely remove and reallocate 
DMA buffers with different size, we have to stop the traffic. So, mana_detach() 
is called. We also call mana_detach() in mana_close(). So the process in 
mana_change_mtu() is no more risky than ifdown/ifup of the NIC. 

In some rare cases, if the system memory is running really low, the bigger 
buffer allocation may fail, so we re-try with the previous MTU. I don't expect 
it to fail again. But we still check & log the error code for completeness and 
debugging.

Thanks,
- Haiyang