Re: [RFC PATCH] net: Fix one page_pool page leak from skb_frag_unref

[Dragos Tatulea <dtatulea@nvidia.com>]

On Fri, 2024-04-26 at 16:05 -0700, Jakub Kicinski wrote:
> On Thu, 25 Apr 2024 08:17:28 +0000 Dragos Tatulea wrote:
> > >  The unref path always dropped a regular page
> > > ref, thanks to this commit as you point out:
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2cc3aeb5ecccec0d266813172fcd82b4b5fa5803
> > > 
> > > AFAICT the correct fix is to actually revert commit 2cc3aeb5eccc
> > > ("skbuff: Fix a potential race while recycling page_pool packets").
> > > The reason is that now that skb_frag_ref() can grab page-pool refs, we
> > > don't need to make sure there is only 1 SKB that triggers the recycle
> > > path anymore. All the skb and its clones can obtain page-pool refs,
> > > and in the unref path we drop the page-pool refs. page_pool_put_page()
> > > detects correctly that the last page-pool ref is put and recycles the
> > > page only then.
> > >   
> > I don't think this is a good way forward. For example, skb->pp_recycle is used
> > as a hint in skb_gro_receive to avoid coalescing skbs with different pp_recycle
> > flag states. This could interfere with that.
> 
> That's a bit speculative, right? The simple invariant we are trying to
> hold is that if skb->pp_recycle && skb_frag_is_pp(skb, i) then the
> reference skb is holding on that frag is a pp reference, not page
> reference.
> 
Yes, it was a speculative statement. After re-reading it and the code of
skb_gro_receive() it makes less sense now.

Mina's suggestion to revert commit 2cc3aeb5eccc ("skbuff: Fix a potential race
while recycling page_pool packets") seems less scary now. I just hope we don't
bump into too many scenarios similar to the ipsec one...

> skb_gro_receive() needs to maintain that invariant, if it doesn't
> we need to fix it..
> 

Thanks,
Dragos