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

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Message-ID: <CAKgT0UfPvkiRSqxOjDUsEVapSbtV++AqSLctZHKKs=_gSxtWfA@mail.gmail.com>
Date:   Wed, 5 Apr 2023 08:06:29 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Liang Chen <liangchen.linux@...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 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.

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))

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.