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Date:   Fri, 4 Aug 2023 11:30:03 +0200
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Subject: Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed
 the packet size limit
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To:     Andrew Kanner <andrew.kanner@gmail.com>,
        Jason Wang <jasowang@redhat.com>
Cc:     davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
        pabeni@redhat.com, netdev@vger.kernel.org, dsahern@gmail.com,
        jbrouer@redhat.com, john.fastabend@gmail.com,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+f817490f5bd20541b90a@syzkaller.appspotmail.com,
        Alexander Duyck <alexander.duyck@gmail.com>
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From:   Jesper Dangaard Brouer <hawk@kernel.org>
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On 03/08/2023 19.53, Andrew Kanner wrote:
>>> Question to Jason Wang:
>>> Why fall back (to e.g. tun_alloc_skb()) when size is above PAGE_SIZE?
>>>
>>> AFAIK tun_build_skb()*can*  create get larger packets than PAGE_SIZE
>>> from it's page_frag.  Is there a reason for this limitation?
>> >> I couldn't recall but I think we can relax.
> > Jesper already sent enough info for this idea in v2, I will use it for
> the next patch/series.
> 

I have some more input and considerations when selecting the new
constant that replace PAGE_SIZE.

Lets see if Eric Dumazet or Alex Duyck disagree?
("inventors" of page_frag scheme)

The function tun_alloc_skb() uses a page_frag scheme for allocation.
The maximim size is 32768 bytes (Order-3), but using something that is
close to this max alloc size can cause memory waste and fragmentation.

My suggestion would be to use the constant SKB_MAX_ALLOC (16KiB).

Maybe Eric or Alex would recommend using something smaller? (e.g. 8192)

page_frag limit comes from:
  #define SKB_FRAG_PAGE_ORDER get_order(32768)


> Jesper, I will add this tag for this next patch/series if you don't
> mind:
> Suggested-by: Jesper Dangaard Brouer <hawk@kernel.org>

ACK