Re: [PATCH 08/13] netvm: Allow skb allocation to use PFMEMALLOC reserves

[NeilBrown <neilb@suse.de>]

On Thu, 28 Apr 2011 11:05:06 +0100 Mel Gorman <mgorman@suse.de> wrote:

> On Thu, Apr 28, 2011 at 04:19:33PM +1000, NeilBrown wrote:
> > On Wed, 27 Apr 2011 17:08:06 +0100 Mel Gorman <mgorman@suse.de> wrote:
> > 
> > 
> > > @@ -1578,7 +1589,7 @@ static inline struct sk_buff *netdev_alloc_skb_ip_align(struct net_device *dev,
> > >   */
> > >  static inline struct page *__netdev_alloc_page(struct net_device *dev, gfp_t gfp_mask)
> > >  {
> > > -	return alloc_pages_node(NUMA_NO_NODE, gfp_mask, 0);
> > > +	return alloc_pages_node(NUMA_NO_NODE, gfp_mask | __GFP_MEMALLOC, 0);
> > >  }
> > >  
> > 
> > I'm puzzling a bit over this change.
> > __netdev_alloc_page appears to be used to get pages to put in ring buffer
> > for a network card to DMA received packets into.  So it is OK to use
> > __GFP_MEMALLOC for these allocations providing we mark the resulting skb as
> > 'pfmemalloc' if a reserved page was used.
> > 
> > However I don't see where that marking is done.
> > I think it should be in skb_fill_page_desc, something like:
> > 
> >   if (page->pfmemalloc)
> > 	skb->pfmemalloc = true;
> > 
> > Is this covered somewhere else that I am missing?
> > 
> 
> You're not missing anything.
> 
> >From the context of __netdev_alloc_page, we do not know if the skb
> is suitable for marking pfmemalloc or not (we don't have SKB_ALLOC_RX
> flag for example that __alloc_skb has). The reserves are potentially
> being dipped into for an unsuitable packet but it gets dropped in
> __netif_receive_skb() and the memory is returned. If we mark the skb
> pfmemalloc as a result of __netdev_alloc_page using a reserve page, the
> packets would not get dropped as expected.
> 

The only code in __netif_receive_skb that seems to drop packets is

+	if (skb_pfmemalloc(skb) && !skb_pfmemalloc_protocol(skb))
+		goto drop;
+

which requires that the skb have pfmemalloc set before it will be dropped.

Actually ... I'm expecting to find code that says:
   if (skb_pfmalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC))
	drop_packet();

but I cannot find it.  Where is the code that discard pfmalloc packets for
non-memalloc sockets?

I can see similar code in sk_filter but that doesn't drop the packet, it just
avoids filtering it.

NeilBrown