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Date:   Wed, 6 Jan 2021 17:49:17 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Willem de Bruijn <willemb@google.com>,
        David Miller <davem@davemloft.net>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Netdev <netdev@vger.kernel.org>
Subject: Re: [BUG] from x86: Support kmap_local() forced debugging
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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..