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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.
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