linux-kernel - Re: [PATCH] tun: avoid high-order page allocation for packet header

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Message-ID: <eab96d92e08f63de5a2321571781faf2343802eb.camel@amazon.com>
Date:   Mon, 31 Jul 2023 23:03:19 +0000
From:   "Erdogan, Tahsin" <trdgn@...zon.com>
To:     "kuba@...nel.org" <kuba@...nel.org>
CC:     "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tun: avoid high-order page allocation for packet header

Thanks Jakub for considering this patch.

On Mon, 2023-07-31 at 13:58 -0700, Jakub Kicinski wrote:
> On Tue, 25 Jul 2023 20:09:36 -0700 Tahsin Erdogan wrote:
> > @@ -1838,6 +1838,9 @@ static ssize_t tun_get_user(struct tun_struct
> > *tun, struct tun_file *tfile,
> >                        */
> >                       zerocopy = false;
> >               } else {
> > +                     if (linear == 0)
> > +                             linear = min_t(size_t, good_linear,
> > copylen);
> 
> nit: would you mind changing to !linear instead of linear == 0 ?

Yes, I will fix this in v2.


> Also - I don't see linear explicitly getting set to 0. What
> guarantees
> that? What's the story there?
linear is set to 0 in the earlier if block. When gso.hdr_len is 0,
linear also becomes 0:
        if (!zerocopy) {
                copylen = len;
                if (tun16_to_cpu(tun, gso.hdr_len) > good_linear)
                        linear = good_linear;
                else
                        linear = tun16_to_cpu(tun, gso.hdr_len);
        }