netdev - Re: [PATCH] openvswitch: Orphan frags before sending to userspace via Netlink to avoid guest stall

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Message-ID: <CALnjE+oMCexif_D3e41F8o9DXn=v6hshj3y-g+bLPoCWtSELww@mail.gmail.com>
Date:	Fri, 7 Mar 2014 10:41:08 -0800
From:	Pravin Shelar <pshelar@...ira.com>
To:	Thomas Graf <tgraf@...hat.com>
Cc:	Zoltan Kiss <zoltan.kiss@...rix.com>,
	Jesse Gross <jesse@...ira.com>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	xen-devel@...ts.xenproject.org, netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] openvswitch: Orphan frags before sending to userspace via
 Netlink to avoid guest stall

On Fri, Mar 7, 2014 at 9:59 AM, Thomas Graf <tgraf@...hat.com> wrote:
> On 03/07/2014 06:28 PM, Pravin Shelar wrote:
>>
>> Problem is mapping SKBTX_DEV_ZEROCOPY pages to userspace. skb_zerocopy
>> is not doing that.
>>
>> Unless I missing something, Current netlink code can not handle
>> skb-frags with zero copy. So we have to copy skb anyways and no need
>> to orphan-frags here.
>> If you are planning on handling skb-frags without copying then
>> skb_orphan_frags should be done in netlink.
>
>
> If you look at the second part of skb_zerocopy() this is exactly what
> it is doing unless the target skb has sufficient linear space
> preallocated. At least unless mmap is enabled in which case we would
> have to copy again until we have implemented a way to pass page refs
> via the nl ring buffer.
>

What is problem with passing page ref with get_page() to new skb as
long as it is accessed kernel?

I think we should set SKBTX_DEV_ZEROCOPY flag for new skb in
skb_zerocopy() to track such pages but no need call
skb_orphan_frags().

> So I think Zoltan is correct in orphaning frags that come from f.e.
> a tun device via zerocopy_sg_from_iovec().
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