netdev - Re: tun: mark small packets as owned by the tap sock

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Message-ID: <20190812221954.GA13314@codemonkey.org.uk>
Date:   Mon, 12 Aug 2019 18:19:54 -0400
From:   Dave Jones <davej@...emonkey.org.uk>
To:     Alexis Bauvin <abauvin@...leway.com>
Cc:     netdev@...r.kernel.org
Subject: Re: tun: mark small packets as owned by the tap sock

On Wed, Aug 07, 2019 at 12:30:07AM +0000, Linux Kernel wrote:
 > Commit:     4b663366246be1d1d4b1b8b01245b2e88ad9e706
 > Parent:     16b2084a8afa1432d14ba72b7c97d7908e178178
 > Web:        https://git.kernel.org/torvalds/c/4b663366246be1d1d4b1b8b01245b2e88ad9e706
 > Author:     Alexis Bauvin <abauvin@...leway.com>
 > AuthorDate: Tue Jul 23 16:23:01 2019 +0200
 > 
 >     tun: mark small packets as owned by the tap sock
 >     
 >     - v1 -> v2: Move skb_set_owner_w to __tun_build_skb to reduce patch size
 >     
 >     Small packets going out of a tap device go through an optimized code
 >     path that uses build_skb() rather than sock_alloc_send_pskb(). The
 >     latter calls skb_set_owner_w(), but the small packet code path does not.
 >     
 >     The net effect is that small packets are not owned by the userland
 >     application's socket (e.g. QEMU), while large packets are.
 >     This can be seen with a TCP session, where packets are not owned when
 >     the window size is small enough (around PAGE_SIZE), while they are once
 >     the window grows (note that this requires the host to support virtio
 >     tso for the guest to offload segmentation).
 >     All this leads to inconsistent behaviour in the kernel, especially on
 >     netfilter modules that uses sk->socket (e.g. xt_owner).
 >     
 >     Fixes: 66ccbc9c87c2 ("tap: use build_skb() for small packet")
 >     Signed-off-by: Alexis Bauvin <abauvin@...leway.com>
 >     Acked-by: Jason Wang <jasowang@...hat.com>

This commit breaks ipv6 routing when I deployed on it a linode.
It seems to work briefly after boot, and then silently all packets get
dropped. (Presumably, it's dropping RA or ND packets)

With this reverted, everything works as it did in rc3.

	Dave