On Thu, 28 Apr 2011 11:05:06 +0100 Mel Gorman <mgorman@xxxxxxx> 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@xxxxxxx> 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
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