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

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Date:   Fri, 23 Sep 2022 11:50:36 -0700
From:   Anirudh Venkataramanan <anirudh.venkataramanan@...el.com>
To:     Alexander Duyck <alexander.duyck@...il.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 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.

> 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.

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