On Fri, 25 Aug 2023 17:40:43 +0800 Yunsheng Lin wrote:

quoted

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.