netdev - Re: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV

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Message-Id: <20151101.155859.1206189858863918018.davem@davemloft.net>
Date:	Sun, 01 Nov 2015 15:58:59 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	haiyangz@...rosoft.com, edumazet@...gle.com,
	netdev@...r.kernel.org, kys@...rosoft.com
Subject: Re: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV_NET is
 enabled

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sun, 01 Nov 2015 09:20:59 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> My recent commit, attaching SYNACK messages to request sockets
> exposed a too small LL_MAX_HEADER when netvsc_drv.c is in use,
> because this driver sets a needed_headroom of 220 bytes.
> 
> Increase LL_MAX_HEADER in this case, to avoid a realloc of all
> TCP frames.
> 
> In another patch, I'll make skb_set_owner_w() more robust.
> 
> Fixes: ca6fb0651883 ("tcp: attach SYNACK messages to request sockets instead of listener")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Bisected-by: Haiyang Zhang <haiyangz@...rosoft.com>

Using a value of 256 just because HYPER-V is crazy imposes a huge
unnecessary burdon upon the rest of the stack.

I rejected a previous attempt to use such a huge value for
LL_MAX_HEADER, and I will do so again here.  We need a different fix
for this issue, one that doesn't hurt everyone.

Every distribution is going to turn all the options on, so you might
as well consider the largest LL_MAX_HEADER value the one %99.999
users end up paying the price for.
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