netdev - Re: [PATCH] skbuff: Fix a race between coalescing and releasing SKBs

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Date:   Thu, 6 Apr 2023 11:28:03 +0800
From:   Liang Chen <liangchen.linux@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, ilias.apalodimas@...aro.org,
        hawk@...nel.org, davem@...emloft.net, edumazet@...gle.com,
        pabeni@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH] skbuff: Fix a race between coalescing and releasing SKBs

On Wed, Apr 5, 2023 at 11:06 PM Alexander Duyck
<alexander.duyck@...il.com> wrote:
>
> On Wed, Apr 5, 2023 at 1:19 AM Liang Chen <liangchen.linux@...il.com> wrote:
> >
> > On Wed, Apr 5, 2023 at 9:21 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > > On Tue, 04 Apr 2023 08:51:18 -0700 Alexander H Duyck wrote:
> > > > I'm not quite sure I agree with the fix. Couldn't we just modify the
> > > > check further down that does:
> > > >
> > > >         if (!skb_cloned(from))
> > > >                 from_shinfo->nr_frags = 0;
> > > >
> > > > And instead just make that:
> > > >       if (!skb->cloned || (!skb_cloned(from) && !from->pp_recycle))
> > > >                 from_shinfo->nr_frags = 0;
> > > >
> > > > With that we would retain the existing behavior and in the case of
> > > > cloned from frames we would take the references and let the original
> > > > from skb freed to take care of pulling the pages from the page pool.
> > >
> > > Sounds like a better fix, indeed. But this sort of code will require
> > > another fat comment above to explain why. This:
> > >
> > >         if (to->pp_recycle == from->pp_recycle && !skb_cloned(from))
> > >
> > > is much easier to understand, no?
> > >
> > > We should at least include that in the explanatory comment, I reckon...
> >
> > Sure, the idea of dealing with the case where @from transitioned into non cloned
> > skb in the function retains the existing behavior, and gives more
> > opportunities to
> > coalesce skbs. And it seems (!skb_cloned(from) && !from->pp_recycle) is enough
> > here.
> > I will take a closer look at the code path for the fragstolen case
> > before making v2
> > patch  -  If @from transitioned into non cloned skb before "if
> > (skb_head_is_locked(from))"
> >
> > Thanks for the reviews.
>
> Actually I am not sure that works now that I look at it closer. The
> problem with using (!skb_cloned(from) && !from->pp_recycle) is that it
> breaks the case where both from and to are pp_recycle without being
> cloned.

Yeah, it would break that case. Thanks!
>
> So it probably needs to be something actually the setup Jakub
> suggested would probably work better:
>   if (to->pp_recycle == from->pp_recycle && !skb_cloned(from))
>

I agree. That's better.
> Basically we just have to guarantee that if we are copying frags over
> the frag destructor is the same for the origin and destination.
> Otherwise we can take a reference and convert them to being reference
> counted.