netdev - Re: SKB paged fragment lifecycle on receive

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Message-ID: <4E051435.8080302@goop.org>
Date:	Fri, 24 Jun 2011 15:48:21 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ian Campbell <Ian.Campbell@...citrix.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xensource.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: SKB paged fragment lifecycle on receive

On 06/24/2011 03:44 PM, Ian Campbell wrote:
> One problem with this is that some functions (__pskb_pull_tail) drop the
> ref count and then remove the page from the skb's fraglist. So by the
> time the destructor is called you have lost the page and cannot do the
> refcount checking.
>
> I suppose we could keep a queue of _all_ pages we ever put in an SKB
> which we poll.

Right, that seems like the only way to make sure we don't lose anything.

>  We could still check for pages with count==1 in the
> destructor. Apart from the other issues with the destructor not being
> copied over clone etc which would cause us to fall-back to polling the
> queue more often than not I reckon.

Yeah, sounds like it.

>> That said, I think an event-based rather than polling based mechanism
>> would be much more preferable.
> Absolutely.

>From what Eric and David were saying, it seems we don't really have any
other workable options.

    J
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