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

[Anirudh Venkataramanan]

On 9/23/2022 8:31 AM, Alexander Duyck wrote:
> On Thu, Sep 22, 2022 at 3:38 PM Anirudh Venkataramanan
> <anirudh.venkataramanan@xxxxxxxxx> wrote:
> > On 9/22/2022 1:58 PM, Alexander Duyck wrote:
> > > On Thu, Sep 22, 2022 at 1:07 PM Anirudh Venkataramanan
> > > <anirudh.venkataramanan@xxxxxxxxx> 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@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
> > >
> > > 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