On Tue, Apr 26, 2011 at 10:21:57PM +1000, NeilBrown wrote:

On Tue, 26 Apr 2011 08:36:50 +0100 Mel Gorman [off-list ref] wrote:

quoted

diff --git a/net/core/dev.c b/net/core/dev.c
index 3871bf6..2d79a20 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -3095,6 +3095,27 @@ static void vlan_on_bond_hook(struct sk_buff *skb)
 	}
 }
 
+/*
+ * Limit which protocols can use the PFMEMALLOC reserves to those that are
+ * expected to be used for communication with swap.
+ */
+static bool skb_pfmemalloc_protocol(struct sk_buff *skb)
+{
+	if (skb_pfmemalloc(skb))
+		switch (skb->protocol) {
+		case __constant_htons(ETH_P_ARP):
+		case __constant_htons(ETH_P_IP):
+		case __constant_htons(ETH_P_IPV6):
+		case __constant_htons(ETH_P_8021Q):
+			break;
+
+		default:
+			return false;
+		}
+
+	return true;
+}

This sort of thing really bugs me :-)
Neither the comment nor the function name actually describe what the function
is doing.  The function is checking *2* things.
   is_pfmemalloc_skb_or_pfmemalloc_protocol()
might be a more correct name, but is too verbose.

I would prefer the skb_pfmemalloc test were removed from here and ....

quoted

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

...added here so this becomes:

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

which actually makes sense.

Moving the check is neater but that check should be

	if (skb_pfmemalloc(skb) && !skb_pfmemalloc_protocol(skb))

? It's only if the skb was allocated from emergency reserves that we
need to consider dropping it to make way for other packets to be
received.

-- 
Mel Gorman
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>