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

[Michael S. Tsirkin]

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@xxxxxxxxxx>
> > 
> > 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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