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

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Date:   Thu, 30 Jun 2022 20:18:36 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     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 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