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From:   Marco Elver <elver@google.com>
Date:   Fri, 27 Nov 2020 13:26:35 +0100
Message-ID: <CANpmjNO8H9OJDTcKhg4PRVEV04Gxnb56mJY2cB9j4cH+4nznhQ@mail.gmail.com>
Subject: Re: [PATCH net-next] net: switch to storing KCOV handle directly in sk_buff
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     Jakub Kicinski <kuba@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Johannes Berg <johannes@sipsolutions.net>,
        Aleksandr Nogikh <a.nogikh@gmail.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        Ido Schimmel <idosch@idosch.org>,
        Florian Westphal <fw@strlen.de>,
        Willem de Bruijn <willemb@google.com>
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On Thu, 26 Nov 2020 at 17:35, Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
> On Thu, Nov 26, 2020 at 3:19 AM Marco Elver <elver@google.com> wrote:
[...]
> > Will send v2.
>
> Does it make more sense to revert the patch that added the extensions
> and the follow-on fixes and add a separate new patch instead?

That doesn't work, because then we'll end up with a build-broken
commit in between the reverts and the new version, because mac80211
uses skb_get_kcov_handle().

> If adding a new field to the skb, even if only in debug builds,
> please check with pahole how it affects struct layout if you
> haven't yet.

Without KCOV:

        /* size: 224, cachelines: 4, members: 72 */
        /* sum members: 217, holes: 1, sum holes: 2 */
        /* sum bitfield members: 36 bits, bit holes: 2, sum bit holes: 4 bits */
        /* forced alignments: 2 */
        /* last cacheline: 32 bytes */

With KCOV:

        /* size: 232, cachelines: 4, members: 73 */
        /* sum members: 225, holes: 1, sum holes: 2 */
        /* sum bitfield members: 36 bits, bit holes: 2, sum bit holes: 4 bits */
        /* forced alignments: 2 */
        /* last cacheline: 40 bytes */


> The skb_extensions idea was mine. Apologies for steering
> this into an apparently unsuccessful direction. Adding new fields
> to skb is very rare because possibly problematic wrt allocation.