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

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Message-ID: <CANn89iJwMOAD_r+4eUpV65PmhMoSHbr0GOE-WA0APZDh3zpiPQ@mail.gmail.com>
Date:   Thu, 6 Apr 2023 11:56:13 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Liang Chen <liangchen.linux@...il.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, ilias.apalodimas@...aro.org,
        hawk@...nel.org, davem@...emloft.net, pabeni@...hat.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH] skbuff: Fix a race between coalescing and releasing SKBs

On Thu, Apr 6, 2023 at 5:28 AM Liang Chen <liangchen.linux@...il.com> wrote:
>
> 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.

Same feeling on my side.
I prefer not trying to merge mixed pp_recycle skbs "just because we
could" at the expense
of adding more code in a fast path.