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

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Date:	Thu, 28 Apr 2011 11:05:06 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	NeilBrown <neilb@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Linux-Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 08/13] netvm: Allow skb allocation to use PFMEMALLOC
 reserves

On Thu, Apr 28, 2011 at 04:19:33PM +1000, NeilBrown wrote:
> On Wed, 27 Apr 2011 17:08:06 +0100 Mel Gorman <mgorman@...e.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.

-- 
Mel Gorman
SUSE Labs
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