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

From: Konrad Rzeszutek Wilk <[email-masked]>
Date: Mon, 13 Aug 2012 11:41:44 -0400

On Wed, Aug 08, 2012 at 03:50:46PM -0700, David Miller wrote:
> From: Mel Gorman <[email-masked]>
> 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 <[email-masked]>
> > Signed-off-by: Mel Gorman <[email-masked]>
> 
> 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.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to [email-masked].  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email-masked]"> [email-masked] </a>