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

[NeilBrown <neilb@suse.de>]

On Thu, 28 Apr 2011 12:18:54 +0100 Mel Gorman <mgorman@suse.de> wrote:

> On Thu, Apr 28, 2011 at 08:47:55PM +1000, NeilBrown wrote:
> > 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.
> > 
> 
> Yes, I only wanted to drop the packet if we were under pressure
> when skb was allocated. If we hit pressure between when skb was
> allocated and when __netdev_alloc_page is called, then the PFMEMALLOC
> reserves may be used for packet receive unnecessarily but the next skb
> allocation that grows slab will have the flag set appropriately. There
> is a window during which we use reserves where we did not have to
> but it's limited. Again, the throttling if pfmemalloc reserves gets too
> depleted comes into play.

I don't find this very convincing...
It seems inconsistent that you are doing precise accounting inside slab so
that you know which object used reserved memory and which did not, yet you
get sloppy with the accounting of whole pages on network receive.

Is there a clear upper bound on how many reserve pages could slip into
non-reserve skbs before skbs start getting the pfmalloc flag set?

I just think it is safer to mark an skb as pfmalloc if any part of the memory
associated with it came from reserves.

Also I find the throttling argument hard to reason about.  Certainly
some things get throttles, but incoming packets don't...

I'm certainly not saying that the code is clearly wrong, but I'm having
trouble convincing myself that it is clearly right (or at least 'safe').

> 
> > 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.
> > 
> 
> hmm, if sk_filter is returning -ENOMEM then things like
> sock_queue_rcv_skb() return error and the skb does not get queued and I
> expected it to get dropped. What did I miss?
> 

Just that I was making incorrect assumptions about code that I wasn't
familiar with.
Make sense now.

Thanks,
NeilBrown

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