netdev - Re: [RFC PATCH] net: Fix one page_pool page leak from skb_frag

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Message-ID: <CAHS8izOsFqiSiS4Z-E-jfD70aogNp5Bcyt7Rk8xFoR2TcDjz=g@mail.gmail.com>
Date: Wed, 1 May 2024 00:48:43 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Dragos Tatulea <dtatulea@...dia.com>
Cc: "kuba@...nel.org" <kuba@...nel.org>, "davem@...emloft.net" <davem@...emloft.net>, 
	"ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"jacob.e.keller@...el.com" <jacob.e.keller@...el.com>, "pabeni@...hat.com" <pabeni@...hat.com>, 
	Jianbo Liu <jianbol@...dia.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"edumazet@...gle.com" <edumazet@...gle.com>
Subject: Re: [RFC PATCH] net: Fix one page_pool page leak from skb_frag_unref

On Tue, Apr 30, 2024 at 11:20 PM Dragos Tatulea <dtatulea@...dia.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.

-- 
Thanks,
Mina