Re: [Intel-wired-lan] [PATCH] ixgbe: Use kmap_local_page in ixgbe_check_lbtest_frame()

From: Alexander Duyck <alexander.duyck@gmail.com>
To: Eric Dumazet <edumazet@google.com>
Cc: Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
	"Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	intel-wired-lan <intel-wired-lan@lists.osuosl.org>,
	Alexander Duyck <alexanderduyck@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Jesse Brandeburg <jesse.brandeburg@intel.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Netdev <netdev@vger.kernel.org>, Jakub Kicinski <kuba@kernel.org>,
	bpf <bpf@vger.kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Ira Weiny <ira.weiny@intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [Intel-wired-lan] [PATCH] ixgbe: Use kmap_local_page in ixgbe_check_lbtest_frame()
Date: Thu, 30 Jun 2022 09:09:18 -0700	[thread overview]
Message-ID: <CAKgT0UcKRJUJrpFHdNrdH98eu_dpiZiVakJRqc2qHrdGJJQRQA@mail.gmail.com> (raw)
In-Reply-To: <CANn89iK6g+4Fy2VMV7=feUAOUDHu-J38be+oU76yp+zGH6xCJQ@mail.gmail.com>

On Thu, Jun 30, 2022 at 8:25 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Thu, Jun 30, 2022 at 5:17 PM Alexander Duyck
> <alexander.duyck@gmail.com> wrote:
> >
> > On Thu, Jun 30, 2022 at 3:10 AM Maciej Fijalkowski
> > <maciej.fijalkowski@intel.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