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

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Date: Fri, 25 Aug 2023 17:08:50 -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 Fri, 25 Aug 2023 17:40:43 +0800 Yunsheng Lin wrote:
> > 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?

IMO you're making this unnecessarily complicated. We can set the masks
in page pool core or just handle the allocation failure like my patch
does and worry about the very unlikely case when someone reports actual
problems.