Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

[Alexander Duyck]

On Wed, Aug 23, 2023 at 7:25 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Wed, 23 Aug 2023 11:03:31 +0800 Yunsheng Lin wrote:
> > On 2023/8/22 23:38, Jakub Kicinski wrote:
> > > On Tue, 22 Aug 2023 17:21:35 +0800 Yunsheng Lin wrote:
> > >> As the CONFIG_PHYS_ADDR_T_64BIT seems to used widely in x86/arm/mips/powerpc,
> > >> I am not sure if we can really make the above assumption.
> > >>
> > >> https://elixir.free-electrons.com/linux/v6.4-rc6/K/ident/CONFIG_PHYS_ADDR_T_64BIT
> > >
> > > Huh, it's actually used a lot less than I anticipated!
> > >
> > > None of the x86/arm/mips/powerpc systems matter IMHO - the only _real_
> >
> > Is there any particular reason that you think that the above systems does
> > not really matter?
>
> Not the systems themselves but the combination of a 32b arch with
> an address space >16TB. All those arches have 64b equivalent, seems
> logical to use the 64b version for a system with a large address space.
> If we're talking about a system which ends up running Linux.
>
> > As we have made a similar wrong assumption about those arches before, I am
> > really trying to be more cautious about it.
> >
> > I searched through the web, some seems to be claiming that "32-bits is DEAD",
> > I am not sure if there is some common agreement among the kernel community,
> > is there any previous discussion about that?
>
> My suspicion/claim is that 32 + PAGE_SHIFT should be enough bits for
> any 32b platform.

One additional thing we could consider would be to simply look at
having page_pool enforce a DMA mask for the device to address any
cases where we might not be able to fit the address. Then in the
unlikely event that somebody is running a 32b system with over 16
terabytes of RAM. With that the DMA subsystem would handle it for us
and we wouldn't have to worry so much about it.