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

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Message-ID: <20230823072552.044d13b3@kernel.org>
Date:   Wed, 23 Aug 2023 07:25:52 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Mina Almasry <almasrymina@...gle.com>, <davem@...emloft.net>,
        <pabeni@...hat.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Liang Chen <liangchen.linux@...il.com>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit
 arch with 64-bit DMA

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.