netdev - Re: [Xen-devel] [PATCH] netvm: check for page == NULL when propogating the skb->pfmemalloc flag

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Message-ID: <20120813154144.GA24868@phenom.dumpdata.com>
Date:	Mon, 13 Aug 2012 11:41:44 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Miller <davem@...emloft.net>,
	Ian Campbell <Ian.Campbell@...citrix.com>
Cc:	mgorman@...e.de, xen-devel@...ts.xensource.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ian.Campbell@...citrix.com, linux-mm@...ck.org, konrad@...nok.org,
	akpm@...ux-foundation.org
Subject: Re: [Xen-devel] [PATCH] netvm: check for page == NULL when
 propogating the skb->pfmemalloc flag

On Wed, Aug 08, 2012 at 03:50:46PM -0700, David Miller wrote:
> From: Mel Gorman <mgorman@...e.de>
> Date: Tue, 7 Aug 2012 09:55:55 +0100
> 
> > Commit [c48a11c7: netvm: propagate page->pfmemalloc to skb] is responsible
> > for the following bug triggered by a xen network driver
>  ...
> > The problem is that the xenfront driver is passing a NULL page to
> > __skb_fill_page_desc() which was unexpected. This patch checks that
> > there is a page before dereferencing.
> > 
> > Reported-and-Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> 
> That call to __skb_fill_page_desc() in xen-netfront.c looks completely bogus.
> It's the only driver passing NULL here.

It looks to be passing a valid page pointer (at least by looking
at the code) so I am not sure how it got turned in a NULL.

But let me double-check by instrumenting the driver..
> 
> That whole song and dance figuring out what to do with the head
> fragment page, depending upon whether the length is greater than the
> RX_COPY_THRESHOLD, is completely unnecessary.
> 
> Just use something like a call to __pskb_pull_tail(skb, len) and all
> that other crap around that area can simply be deleted.

It looks like an overkill - it does a lot more than just allocate an SKB
and a page.

Deleting of extra code would be nice - however I am not going to be able
to do that for the next two weeks sadly - as my plate if full of debugging
some other stuff.

Lets see if Ian has some time.
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