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From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
To: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>,
	Netdev <netdev@vger.kernel.org>,
	Alexander Duyck <alexanderduyck@fb.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Alexander Lobakin <alobakin@pm.me>,
	Jonathan Lemon <jonathan.lemon@gmail.com>,
	Willem de Bruijn <willemb@google.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Guillaume Nault <gnault@redhat.com>,
	Cong Wang <cong.wang@bytedance.com>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Matteo Croce <mcroce@microsoft.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/1 v2] skbuff: Fix a potential race while recycling page_pool packets
Date: Thu, 15 Jul 2021 17:45:41 +0300	[thread overview]
Message-ID: <YPBKFXWdDytvPmoN@Iliass-MBP> (raw)
In-Reply-To: <CAKgT0UemhFPHo9krmQfm=yNTSjwpBwVkoFtLEEQ-qLVh=-BeHg@mail.gmail.com>

> > >           atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,

[...]

> > >                             &shinfo->dataref))
> > > -             return;
> > > +             goto exit;
> >
> > Is it possible this patch may break the head frag page for the original skb,
> > supposing it's head frag page is from the page pool and below change clears
> > the pp_recycle for original skb, causing a page leaking for the page pool?
> 
> I don't see how. The assumption here is that when atomic_sub_return
> gets down to 0 we will still have an skb with skb->pp_recycle set and
> it will flow down and encounter skb_free_head below. All we are doing
> is skipping those steps and clearing skb->pp_recycle for all but the
> last buffer and the last one to free it will trigger the recycling.

I think the assumption here is that 
1. We clone an skb
2. The original skb goes into pskb_expand_head()
3. skb_release_data() will be called for the original skb

But with the dataref bumped, we'll skip the recycling for it but we'll also
skip recycling or unmapping the current head (which is a page_pool mapped
buffer)

> 
> > >
> > >       skb_zcopy_clear(skb, true);
> > >
> > > @@ -674,6 +674,8 @@ static void skb_release_data(struct sk_buff *skb)
> > >               kfree_skb_list(shinfo->frag_list);
> > >
> > >       skb_free_head(skb);
> > > +exit:
> > > +     skb->pp_recycle = 0;
> 
> Note the path here. We don't clear skb->pp_recycle for the last buffer
> where "dataref == 0" until *AFTER* the head has been freed, and all
> clones will have skb->pp_recycle = 1 as long as they are a clone of
> the original skb that had it set.

  reply	other threads:[~2021-07-15 14:45 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-09  6:29 Ilias Apalodimas
2021-07-09 14:34 ` Alexander Duyck
2021-07-09 17:29   ` Ilias Apalodimas
2021-07-12 11:52   ` Jesper Dangaard Brouer
2021-07-15  4:00 ` Yunsheng Lin
2021-07-15 10:00   ` Ilias Apalodimas
2021-07-15 10:38     ` Ilias Apalodimas
2021-07-15 10:47       ` Yunsheng Lin
2021-07-15 12:37         ` Ilias Apalodimas
2021-07-15 14:25   ` Alexander Duyck
2021-07-15 14:45     ` Ilias Apalodimas [this message]
2021-07-15 14:57       ` Alexander Duyck
2021-07-15 15:02         ` Ilias Apalodimas
2021-07-16  2:30           ` Yunsheng Lin
2021-07-16  6:43             ` Ilias Apalodimas

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