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@suse.de>
Date: Tue, 7 Aug 2012 09:55:55 +0100

quoted

Commit [c48a11c7: netvm: propagate page->pfmemalloc to skb] is responsible
for the following bug triggered by a xen network driver

 ...

quoted

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 <redacted>
Signed-off-by: Mel Gorman <mgorman@suse.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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