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

On Wed, Jun 23, 2021 at 5:07 PM Jakub Kicinski [off-list ref] wrote:

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

quoted

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.

The 15 seems to come from alignment indeed. Not sure when it was
introduced, but until 56951b54e87a there is also this

                /*
                 *      Get the memory we require with some space left
for alignment.
                 */

                skb = sock_alloc_send_skb(sk, fraglen+hh_len+15, 0,
flags&MSG_DONTWAIT, &err);