Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA
From: Yunsheng Lin <hidden>
Date: 2023-08-25 09:41:46
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On 2023/8/24 2:00, Alexander Duyck wrote:
On Wed, Aug 23, 2023 at 7:25 AM Jakub Kicinski [off-list ref] wrote:quoted
On Wed, 23 Aug 2023 11:03:31 +0800 Yunsheng Lin wrote:quoted
On 2023/8/22 23:38, Jakub Kicinski wrote:quoted
On Tue, 22 Aug 2023 17:21:35 +0800 Yunsheng Lin wrote:quoted
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_64BITHuh, 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.quoted
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.
It seems there is a API to acquire the DMA mask used by the device: https://elixir.free-electrons.com/linux/v6.4-rc6/source/include/linux/dma-mapping.h#L434 Is it possible to use that to check if DMA mask used by the device is within 32 + PAGE_SHIFT limit, if yes, we use jakub's proposal to reduce reduce the dma address bit, if no, we fail the page_pool creation?
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