Date:   Fri, 23 Sep 2022 14:31:03 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Anirudh Venkataramanan <anirudh.venkataramanan@...el.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Netdev <netdev@...r.kernel.org>,
        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>, bpf <bpf@...r.kernel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Ira Weiny <ira.weiny@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Tony Nguyen <anthony.l.nguyen@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH] ixgbe: Use kmap_local_page in ixgbe_check_lbtest_frame()

On Fri, Sep 23, 2022 at 11:51 AM Anirudh Venkataramanan
<anirudh.venkataramanan@...el.com> wrote:
>
> On 9/23/2022 8:31 AM, Alexander Duyck wrote:
> > On Thu, Sep 22, 2022 at 3:38 PM Anirudh Venkataramanan
> > <anirudh.venkataramanan@...el.com> wrote:
> >>
> >> On 9/22/2022 1:58 PM, Alexander Duyck wrote:
> >>> On Thu, Sep 22, 2022 at 1:07 PM Anirudh Venkataramanan
> >>> <anirudh.venkataramanan@...el.com> wrote:
> >>>>
> >>>>
> >>>> Following Fabio's patches, I made similar changes for e1000/e1000e and
> >>>> submitted them to IWL [1].
> >>>>
> >>>> Yesterday, Ira Weiny pointed me to some feedback from Dave Hansen on the
> >>>> use of page_address() [2]. My understanding of this feedback is that
> >>>> it's safer to use kmap_local_page() instead of page_address(), because
> >>>> you don't always know how the underlying page was allocated.
> >>>>
> >>>> This approach (of using kmap_local_page() instead of page_address())
> >>>> makes sense to me. Any reason not to go this way?
> >>>>
> >>>> [1]
> >>>>
> >>>> https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20220919180949.388785-1-anirudh.venkataramanan@intel.com/
> >>>>
> >>>> https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20220919180949.388785-2-anirudh.venkataramanan@intel.com/
> >>>>
> >>>> [2]
> >>>> https://lore.kernel.org/lkml/5d667258-b58b-3d28-3609-e7914c99b31b@intel.com/
> >>>>
> >>>> Ani
> >>>
> >>> For the two patches you referenced the driver is the one allocating
> >>> the pages. So in such a case the page_address should be acceptable.
> >>> Specifically we are falling into alloc_page(GFP_ATOMIC) which should
> >>> fall into the first case that Dave Hansen called out.
> >>
> >> Right. However, I did run into a case in the chelsio inline crypto
> >> driver where it seems like the pages are allocated outside the driver.
> >> In such cases, kmap_local_page() would be the right approach, as the
> >> driver can't make assumptions on how the page was allocated.
> >
> > Right, but that is comparing apples and oranges. As I said for Tx it
> > would make sense, but since we are doing the allocations for Rx that
> > isn't the case so we don't need it.
> >
> >> ... and this makes me wonder why not just use kmap_local_page() even in
> >> cases where the page allocation was done in the driver. IMO, this is
> >> simpler because
> >>
> >> a) you don't have to care how a page was allocated. kmap_local_page()
> >> will create a temporary mapping if required, if not it just becomes a
> >> wrapper to page_address().
> >>
> >> b) should a future patch change the allocation to be from highmem, you
> >> don't have to change a bunch of page_address() calls to be
> >> kmap_local_page().
> >>
> >> Is using page_address() directly beneficial in some way?
> >
> > By that argument why don't we just leave the code alone and keep using
> > kmap? I am pretty certain that is the logic that had us using kmap in
> > the first place since it also dumps us with page_address in most cases
> > and we didn't care much about the other architectures.
>
> Well, my understanding is that kmap_local_page() doesn't have the
> overheads kmap() has, and that alone is reason enough to replace kmap()
> and kmap_atomic() with kmap_local_page() where possible.

It has less overhead, but there is still some pretty significant code
involved. Basically in the cases where it can't bail out and just call
page_address it will call __kmap_local_page_prot(),
https://elixir.bootlin.com/linux/v6.0-rc4/source/mm/highmem.c#L517.

> > If you look at
> > the kmap_local_page() it just adds an extra step or two to calling
> > page_address(). In this case it is adding extra complication to
> > something that isn't needed which is the reason why we are going
> > through this in the first place. If we are going to pull the bandage I
> > suggest we might as well just go all the way and not take a half-step
> > since we don't actually need kmap or its related calls for this.
>
> I don't really see this as "pulling the kmap() bandage", but a "use a
> more appropriate kmap function if you can" type situation.

My concern is that it is more of a half step in the case of the
e1000/e1000e drivers. We likely should have fixed this some time ago
when I had rewritten the Rx path for the igb and ixgbe drivers, but I
just didn't get around to it because if I messed with other areas it
would have required more validation. I'd rather not carry around the
extra code or function calls if we don't need it.

> FWIW, I am not against using page_address(). Just wanted to hash this
> out and get to a conclusion before I made new changes.
>
> Ani

I gathered as much based on your other conversation. This is
essentially the module-local case you had referred to in which the
page is allocated and used within the module so there is no need to be
concerned about it possibly being a highmem page.

Thanks,

- Alex

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