netdev - Re: [PATCH] net: correct zerocopy refcnt with newly allocated UDP or RAW uarg

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Date:   Sat, 15 Aug 2020 02:02:18 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC:     David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        "Jakub Kicinski" <kuba@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: correct zerocopy refcnt with newly allocated UDP or
 RAW uarg

Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
>On Fri, Aug 14, 2020 at 10:17 AM linmiaohe <linmiaohe@...wei.com> wrote:
>>
>
>I don't think that can happen.
>
>The question is when this branch is false
>
>                next = (u32)atomic_read(&sk->sk_zckey);
>                if ((u32)(uarg->id + uarg->len) == next) {
>
>I cannot come up with a case. I think it might be vestigial. The goal is to ensure to append only a consecutive range of notification IDs.
>Each notification ID corresponds to a sendmsg invocation with MSG_ZEROCOPY. In both TCP and UDP with corking, data is ordered and access to changes to these fields happen together as a transaction:
>
>                /* realloc only when socket is locked (TCP, UDP cork),
>                 * so uarg->len and sk_zckey access is serialized
>                 */

So what I concerned is just a theoretically problems. If we can always guarantee sock_zerocopy_realloc only returns the existing uarg or NULL when on skb_zcopy(skb),
bad things won't happen.

Many thanks for your detailed explaination.