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Date:   Wed, 6 Jan 2021 17:49:17 -0800
From:   Jakub Kicinski <>
To:     Linus Torvalds <>
Cc:     Steven Rostedt <>,
        Willem de Bruijn <>,
        David Miller <>,
        Jonathan Lemon <>,
        Thomas Gleixner <>,
        LKML <>,
        "the arch/x86 maintainers" <>,
        Christoph Hellwig <>,
        Matthew Wilcox <>,
        Daniel Vetter <>,
        Andrew Morton <>,
        Linux-MM <>,
        Peter Zijlstra <>,
        Ingo Molnar <>,
        Juri Lelli <>,
        Vincent Guittot <>,
        Dietmar Eggemann <>,
        Ben Segall <>, Mel Gorman <>,
        Daniel Bristot de Oliveira <>,
        Netdev <>
Subject: Re: [BUG] from x86: Support kmap_local() forced debugging

On Wed, 6 Jan 2021 17:03:48 -0800 Linus Torvalds wrote:
> I wonder whether there is other code that "knows" about kmap() only
> affecting PageHighmem() pages thing that is no longer true.
> Looking at some other code, skb_gro_reset_offset() looks suspiciously
> like it also thinks highmem pages are special.
> Adding the networking people involved in this area to the cc too.

Thanks for the detailed analysis! skb_gro_reset_offset() checks if
kernel can read data in the fragments directly as an optimization, 
in case the entire header is in a fragment. 

IIUC DEBUG_KMAP_LOCAL_FORCE_MAP only affects the mappings from
explicit kmap calls, which GRO won't make - it will fall back to
pulling the header out of the fragment and end up in skb_copy_bits(),
i.e. the loop you fixed. So GRO should be good. I think..

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