linux-kernel - Re: [PATCH 2/6] macvtap: zerocopy: fix truesize underestimation

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Message-ID: <20120416132520.GA13113@redhat.com>
Date:	Mon, 16 Apr 2012 16:25:20 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
	xma@...ibm.com, davem@...emloft.net, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com
Subject: Re: [PATCH 2/6] macvtap: zerocopy: fix truesize underestimation

On Mon, Apr 16, 2012 at 10:49:53AM +0200, Eric Dumazet wrote:
> On Mon, 2012-04-16 at 10:14 +0300, Michael S. Tsirkin wrote:
> > On Mon, Apr 16, 2012 at 02:07:59PM +0800, Jason Wang wrote:
> > > As the skb fragment were pinned/built from user pages, we should
> > > account the page instead of length for truesize.
> > > 
> > > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > 
> > I'm not sure this is right: the skb does *not* consume the
> > whole page, userspace uses the rest of the page
> > for other skbs. So we'll end up accounting for the
> > same page twice.
> > Eric, what's the right thing to do here in your opinion?
> 
> Problem is we dont know for sure userspace wont free pages right after
> this syscall. So an evil application could consume more kernel memory
> than what socket limit allowed.
> 
> Its same problem with vmsplice(mem -> pipe) + splice(pipe -> socket)
> 
> When we clone skb with frags, resulting skb will have same truesize,
> even if the pages are shared ...
> 

I see, thanks for the clarification.

-- 
MST
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