Messages in this thread | | | Date | Fri, 4 Aug 2023 11:30:03 +0200 | Subject | Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit | From | Jesper Dangaard Brouer <> |
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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
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