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

Dragos Tatulea
2024-05-01T07:58:07Z

On Wed, 2024-05-01 at 00:48 -0700, Mina Almasry wrote:
> On Tue, Apr 30, 2024 at 11:20 PM Dragos Tatulea <dtatulea@nvidia.com> wrote:
> >
> > On Mon, 2024-04-29 at 09:39 +0200, Dragos Tatulea wrote:
> > > 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..
> > > >
> > >
> > Gentle ping. Not sure how to proceed with this:
> >
> > 1) Revert commit 2cc3aeb5eccc
> > ("skbuff: Fix a potential race while recycling page_pool packets"). I tested
> > this btw and it works (for this specific scenario).
> >
> > 2) Revert Mina's commit a580ea994fd3 ("net: mirror skb frag ref/unref helpers")
> > for now.
> >
>
> I vote for #1, and IIUC Jakub's feedback, he seems to prefer this as
> well. If we continue to run into edge cases after the revert of #1, I
> think we may want to do #2 and I can look to reland it with the kunit
> tests that Jakub suggested that reproduce these edge cases.
>
> I can upload #1 in the morning if there are no objections. I don't see
> any regressions with #1 but I was never able to repo this issue.
>
Yes please. And once you post it we can also take it internally to check all the
other tests that were failing in the same place.

Thanks,
Dragos