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

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Date:   Thu, 30 Jun 2022 20:13:37 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     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 giovedì 30 giugno 2022 17:17:24 CEST Alexander Duyck wrote:
> 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

Please take a look at documentation (highmem.rst). I've recently reworked 
it and added information about kmap_local_page()

Thanks,

Fabio

> 
> Thanks,
> 
> - Alex
>