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From:   Ilias Apalodimas <ilias.apalodimas@linaro.org>
Date:   Fri, 16 Jul 2021 09:43:53 +0300
Message-ID: <CAC_iWjL7X+wmeSk8m0q5=MdrPP-NiZY3K7RjCE6_GM2CxfN_7g@mail.gmail.com>
Subject: Re: [PATCH 1/1 v2] skbuff: Fix a potential race while recycling
 page_pool packets
To:     Yunsheng Lin <linyunsheng@huawei.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.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>
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Precedence: bulk

On Fri, 16 Jul 2021 at 05:30, Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> On 2021/7/15 23:02, Ilias Apalodimas wrote:
> > On Thu, Jul 15, 2021 at 07:57:57AM -0700, Alexander Duyck wrote:
> >> On Thu, Jul 15, 2021 at 7:45 AM Ilias Apalodimas
> >> <ilias.apalodimas@linaro.org> wrote:
> >>>
> >>>>>>           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)
> >>
> >> Right, but in that case it is the clone that is left holding the
> >> original head and the skb->pp_recycle flag is set on the clone as it
> >> was copied from the original when we cloned it.
> >
> > Ah yes, that's what I missed
> >
> >> What we have
> >> essentially done is transferred the responsibility for freeing it from
> >> the original to the clone.
> >>
> >> If you think about it the result is the same as if step 2 was to go
> >> into kfree_skb. We would still be calling skb_release_data and the
> >> dataref would be decremented without the original freeing the page. We
> >> have to wait until all the clones are freed and dataref reaches 0
> >> before the head can be recycled.
> >
> > Yep sounds correct
>
> Ok, I suppose the fraglist skb is handled similar as the regular skb, right?
>

Yes, even in the fragments case your cloned/expanded SBK will still
have the recycle bit set, so it will try to recycle them or unmap them

> Also, this patch might need respinning as the state of this patch is "Changes
> Requested" in patchwork.

Thanks, I'll respin it and add a comment explaining why

>
> >
> > Thanks
> > /Ilias
> > .
> >