netdev - Re: [PATCH net v2 1/2] gianfar: Replace skb_realloc_headroom with skb_cow

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Message-ID: <20201103171849.vthpu7beenzeayrs@skbuf>
Date:   Tue, 3 Nov 2020 19:18:49 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Julian Wiedmann <jwi@...ux.ibm.com>
Cc:     Claudiu Manoil <claudiu.manoil@....com>, netdev@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, james.jurack@...tek.com
Subject: Re: [PATCH net v2 1/2] gianfar: Replace skb_realloc_headroom with
 skb_cow_head for PTP

On Tue, Nov 03, 2020 at 06:36:30PM +0200, Julian Wiedmann wrote:
> Given the various skb modifications in its xmit path, I wonder why
> gianfar doesn't clear IFF_TX_SKB_SHARING.

Thanks for the hint Julian :) I'll try to see if it makes any
difference.

Just a wild guess, but maybe because the gianfar maintainer (or me) had
no idea about the existence of the flag, and because IFF_SKB_TX_SHARED
was added _after_ gianfar was already a thing (which it was since the
beginning of git), which is insane to begin with?

commit d8873315065f1f527c7c380402cf59b1e1d0ae36
Author: Neil Horman <nhorman@...driver.com>
Date:   Tue Jul 26 06:05:37 2011 +0000

    net: add IFF_SKB_TX_SHARED flag to priv_flags

    Pktgen attempts to transmit shared skbs to net devices, which can't be used by
    some drivers as they keep state information in skbs.  This patch adds a flag
    marking drivers as being able to handle shared skbs in their tx path.  Drivers
    are defaulted to being unable to do so, but calling ether_setup enables this
    flag, as 90% of the drivers calling ether_setup touch real hardware and can
    handle shared skbs.  A subsequent patch will audit drivers to ensure that the
    flag is set properly

    Signed-off-by: Neil Horman <nhorman@...driver.com>
    Reported-by: Jiri Pirko <jpirko@...hat.com>
    CC: Robert Olsson <robert.olsson@....uu.se>
    CC: Eric Dumazet <eric.dumazet@...il.com>
    CC: Alexey Dobriyan <adobriyan@...il.com>
    CC: David S. Miller <davem@...emloft.net>
    Signed-off-by: David S. Miller <davem@...emloft.net>