linux-kernel - Re: [PATCH net] hv_netvsc: Restore needed

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Message-Id: <20160213.060507.1405887572923400829.davem@davemloft.net>
Date:	Sat, 13 Feb 2016 06:05:07 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	vkuznets@...hat.com
Cc:	netdev@...r.kernel.org, devel@...uxdriverproject.org,
	linux-kernel@...r.kernel.org, kys@...rosoft.com,
	haiyangz@...rosoft.com
Subject: Re: [PATCH net] hv_netvsc: Restore needed_headroom request

From: Vitaly Kuznetsov <vkuznets@...hat.com>
Date: Fri,  5 Feb 2016 17:29:08 +0100

> Commit c0eb454034aa ("hv_netvsc: Don't ask for additional head room in the
> skb") got rid of needed_headroom setting for the driver. With the change I
> hit the following issue trying to use ptkgen module:
> 
> [   57.522021] kernel BUG at net/core/skbuff.c:1128!
> [   57.522021] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC
> ...
> [   58.721068] Call Trace:
> [   58.721068]  [<ffffffffa0144e86>] netvsc_start_xmit+0x4c6/0x8e0 [hv_netvsc]
> ...
> [   58.721068]  [<ffffffffa02f87fc>] ? pktgen_finalize_skb+0x25c/0x2a0 [pktgen]
> [   58.721068]  [<ffffffff814f5760>] ? __netdev_alloc_skb+0xc0/0x100
> [   58.721068]  [<ffffffffa02f9907>] pktgen_thread_worker+0x257/0x1920 [pktgen]
> 
> Basically, we're calling skb_cow_head(skb, RNDIS_AND_PPI_SIZE) and crash on
>     if (skb_shared(skb))
>         BUG();
> 
> We probably need to restore needed_headroom setting (but shrunk to
> RNDIS_AND_PPI_SIZE as we don't need more) to request the required headroom
> space. In theory, it should not give us performance penalty.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Applied, thanks.