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

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Date:   Thu, 22 Sep 2022 13:07:22 -0700
From:   Anirudh Venkataramanan <anirudh.venkataramanan@...el.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Netdev <netdev@...r.kernel.org>
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>, 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 7/1/2022 8:36 AM, Fabio M. De Francesco wrote:
> On giovedì 30 giugno 2022 23:59:23 CEST Alexander Duyck wrote:
>> 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
>>
> 
> OK, I'm about to prepare another patch with page_address() (obviously, this
> should be discarded).
> 
> Last thing... Is that page allocated with dma_pool_alloc() at
> ixgbe/ixgbe_fcoe.c:196? Somewhere else?
> 
> Thanks,
> 
> Fabio
> 
> P.S.: Can you say something about how pages are allocated in intel/e1000
> and in intel/e1000e? I see that those drivers use kmap_atomic().

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