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From:   Taehee Yoo <ap420073@gmail.com>
Date:   Wed, 2 Oct 2019 03:23:52 +0900
Message-ID: <CAMArcTX7RLaqxYXMfKbC5Kw0JdXauwaxRQGxh=bZqqYafYAPLQ@mail.gmail.com>
Subject: Re: [PATCH net v4 01/12] net: core: limit nested device depth
To:     Johannes Berg <johannes@sipsolutions.net>
Cc:     David Miller <davem@davemloft.net>,
        Netdev <netdev@vger.kernel.org>, linux-wireless@vger.kernel.org,
        Jakub Kicinski <jakub.kicinski@netronome.com>,
        j.vosburgh@gmail.com, vfalico@gmail.com,
        Andy Gospodarek <andy@greyhouse.net>,
        =?UTF-8?B?SmnFmcOtIFDDrXJrbw==?= <jiri@resnulli.us>,
        sd@queasysnail.net, Roopa Prabhu <roopa@cumulusnetworks.com>,
        saeedm@mellanox.com, manishc@marvell.com, rahulv@marvell.com,
        kys@microsoft.com, haiyangz@microsoft.com,
        Stephen Hemminger <stephen@networkplumber.org>,
        sashal@kernel.org, hare@suse.de, varun@chelsio.com,
        ubraun@linux.ibm.com, kgraul@linux.ibm.com,
        Jay Vosburgh <jay.vosburgh@canonical.com>,
        Cody Schuffelen <schuffelen@google.com>, bjorn@mork.no
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On Tue, 1 Oct 2019 at 22:57, Johannes Berg <johannes@sipsolutions.net> wrote:
>
> Hi,
>

Hi!

> (jumping out now, forgive me for being so brief)
>
> > If I understand correctly, you said about the alignment of
> > "lower_level" and "upper_level".
> > I thought this place is a fine position for variables as regards the
> > alignment and I didn't try to put each variable in different places.
> >
> > If I misunderstood your mention, please let me know.
>
> Not sure what you mean, alignment doesn't matter for them (they're u8).
>
> I was thinking of the packing for the overall struct, we have:
>
>         unsigned int            max_mtu;
>         unsigned short          type;
>         unsigned short          hard_header_len;
>         unsigned char           min_header_len;
>
> +       unsigned char           upper_level, lower_level;
>
>         unsigned short          needed_headroom;
>         unsigned short          needed_tailroom;
>
>
> Previously, there was a one byte hole at that spot due to a single
> "unsigned char" (after something aligned at least 4 bytes) followed by
> "unsigned short" - now you push that out a bit.
>
> If you place the variables a bit lower, below "name_assign_type", you
> probably fill a hole instead.
>
> Check out the 'pahole' tool.
>

Thank you for the detailed explanation.
I tested the pahole and found holes.

$ pahole ./vmlinux.o -C net_device
        unsigned char              addr_assign_type;     /*   598     1 */
        unsigned char              addr_len;             /*   599     1 */
        short unsigned int         neigh_priv_len;       /*   600     2 */
        short unsigned int         dev_id;               /*   602     2 */
        short unsigned int         dev_port;             /*   604     2 */

        /* XXX 2 bytes hole, try to pack */

I will place the variables here.

> johannes
>

Thank you so much!
Taehee