Re: [PATCH] tcp: splice as many packets as possible at once

David Miller
2009-01-14T07:27:24Z

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Wed, 14 Jan 2009 14:51:24 +1100

> Unfortunately this won't work, not even for network destinations.
>
> The reason is that this gets called as soon as the destination's
> splice hook returns, for networking that means when sendpage returns.
>
> So by that time we'll still be left with just a page reference
> on a page where the slab memory may already have been freed.
>
> To make this work we need to get the destination's splice hooks
> to acquire this reference.

So while trying to figure out a sane way to fix this, I found
another bug:

/*
* map the linear part
*/
if (__splice_segment(virt_to_page(skb->data),
(unsigned long) skb->data & (PAGE_SIZE - 1),
skb_headlen(skb),
offset, len, skb, spd))
return 1;

This will explode if the SLAB cache for skb->head is using compound
(ie. order > 0) pages.

For example, if this is an order-1 page being used for the skb->head
data (which would be true on most systems for jumbo MTU frames being
received into a linear SKB), the offset will be wrong and depending
upon skb_headlen() we could reference past the end of that
non-compound page we will end up grabbing a reference to.

And then we'll end up with a compound page in an skb_shinfo() frag
array, which is illegal.

Well, at least, I can list several drivers that will barf when
trying to TX that (Acenic, atlx1, cassini, jme, sungem), since
they use pci_map_page(... virt_to_page(skb->data)) or similar.

The core KMAP'ing support for SKBs will also not be able to grok
such a beastly SKB.