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

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Date:   Thu, 22 Sep 2022 15:38:02 -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 9/22/2022 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.

... 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?

Ani