Re: [PATCH net-next v3] net: ip: avoid OOM kills with large UDP sends over loopback

On Wed, 23 Jun 2021 21:45:55 +0200 Jesper Dangaard Brouer wrote:

quoted

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index c3efc7d658f6..790dd28fd198 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c

@@ -1077,7 +1077,9 @@ static int __ip_append_data(struct sock *sk,
 			if ((flags & MSG_MORE) &&
 			    !(rt->dst.dev->features&NETIF_F_SG))
 				alloclen = mtu;
-			else if (!paged)
+			else if (!paged &&
+				 (fraglen + hh_len + 15 < SKB_MAX_ALLOC ||  

What does the number 15 represent here?

No idea, it's there on the allocation line, so I need to include it on
the size check.

Looking at super old code (2.4.x) it looks like it may have gotten
copy & pasted mistakenly? The hard headers are rounded up to 16B,
and there is code which does things like:

	skb_alloc(size + dev->hard_header_len + 15);
	skb_reserve(skb, (dev->hard_header_len + 15) & ~15);

in other spots. So if I was to guess I'd say someone decided to add the
15B "to be safe" even though hh_len already includes the round up here.

But that's just my guess. I can't get this simple patch right,
so take that with a grain of salt :/

quoted

+				  !(rt->dst.dev->features & NETIF_F_SG)))
 				alloclen = fraglen;
 			else {
 				alloclen = min_t(int, fraglen, MAX_HEADER);

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index ff4f9ebcf7f6..ae8dbd6cdab1 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c

@@ -1585,7 +1585,9 @@ static int __ip6_append_data(struct sock *sk,
 			if ((flags & MSG_MORE) &&
 			    !(rt->dst.dev->features&NETIF_F_SG))
 				alloclen = mtu;
-			else if (!paged)
+			else if (!paged &&
+				 (fraglen + hh_len < SKB_MAX_ALLOC ||  

The number 15 is not use here.

quoted

+				  !(rt->dst.dev->features & NETIF_F_SG)))
 				alloclen = fraglen;
 			else {
 				alloclen = min_t(int, fraglen, MAX_HEADER);