linux-kernel - Re: [Intel-wired-lan] [PATCH] ixgbe: Use kmap_local_page in ixgbe_check_lbtest

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Date:   Thu, 30 Jun 2022 08:17:24 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        Alexander Duyck <alexanderduyck@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Netdev <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>, bpf <bpf@...r.kernel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Ira Weiny <ira.weiny@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Intel-wired-lan] [PATCH] ixgbe: Use kmap_local_page in ixgbe_check_lbtest_frame()

On Thu, Jun 30, 2022 at 3:10 AM Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> On Wed, Jun 29, 2022 at 10:58:36AM +0200, Fabio M. De Francesco wrote:
> > The use of kmap() is being deprecated in favor of kmap_local_page().
> >
> > With kmap_local_page(), the mapping is per thread, CPU local and not
> > globally visible. Furthermore, the mapping can be acquired from any context
> > (including interrupts).
> >
> > Therefore, use kmap_local_page() in ixgbe_check_lbtest_frame() because
> > this mapping is per thread, CPU local, and not globally visible.
>
> Hi,
>
> I'd like to ask why kmap was there in the first place and not plain
> page_address() ?
>
> Alex?

The page_address function only works on architectures that have access
to all of physical memory via virtual memory addresses. The kmap
function is meant to take care of highmem which will need to be mapped
before it can be accessed.

For non-highmem pages kmap just calls the page_address function.
https://elixir.bootlin.com/linux/latest/source/include/linux/highmem-internal.h#L40

Thanks,

- Alex