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

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Message-ID: <CAKgT0UfThk3MLcE38wQu5+2Qy7Ld2px-2WJgnD+2xbDsA8iEEw@mail.gmail.com>
Date:   Thu, 30 Jun 2022 14:59:23 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.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>,
        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 11:18 AM Fabio M. De Francesco
<fmdefrancesco@...il.com> wrote:
>
> On giovedì 30 giugno 2022 18:09:18 CEST Alexander Duyck wrote:
> > On Thu, Jun 30, 2022 at 8:25 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > On Thu, Jun 30, 2022 at 5:17 PM Alexander Duyck
> > > <alexander.duyck@...il.com> 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
> > >
> > >
> > > Sure, but drivers/net/ethernet/intel/ixgbe/ixgbe_main.c is allocating
> > > pages that are not highmem ?
> > >
> > > This kmap() does not seem needed.
> >
> > Good point. So odds are page_address is fine to use. Actually there is
> > a note to that effect in ixgbe_pull_tail.
> >
> > As such we could probably go through and update igb, and several of
> > the other Intel drivers as well.
> >
> > - Alex
> >
> I don't know this code, however I know kmap*().
>
> I assumed that, if author used kmap(), there was possibility that the page
> came from highmem.
>
> In that case kmap_local_page() looks correct here.
>
> However, now I read that that page _cannot_ come from highmem. Therefore,
> page_address() would suffice.
>
> If you all want I can replace kmap() / kunmap() with a "plain"
> page_address(). Please let me know.
>
> Thanks,
>
> Fabio

Replacing it with just page_address() should be fine. Back when I
wrote the code I didn't realize that GFP_ATOMIC pages weren't
allocated from highmem so I suspect I just used kmap since it was the
way to cover all the bases.

Thanks,

- Alex