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

[Anirudh Venkataramanan]

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@xxxxxxxxx> 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@xxxxxxxxxx>
> wrote:
> > > > > On Thu, Jun 30, 2022 at 5:17 PM Alexander Duyck
> > > > > <alexander.duyck@xxxxxxxxx> wrote:
> > > > > > On Thu, Jun 30, 2022 at 3:10 AM Maciej Fijalkowski
> > > > > > <maciej.fijalkowski@xxxxxxxxx> 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@xxxxxxxxx/

https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20220919180949.388785-2-anirudh.venkataramanan@xxxxxxxxx/

[2] 
https://lore.kernel.org/lkml/5d667258-b58b-3d28-3609-e7914c99b31b@xxxxxxxxx/

Ani