Re: [PATCH 04/35] dmem: let pat recognize dmem

[Paolo Bonzini]

On 08/10/20 09:53, yulei.kernel@xxxxxxxxx wrote:
> From: Yulei Zhang <yuleixzhang@xxxxxxxxxxx>
> 
> x86 pat uses 'struct page' by only checking if it's system ram,
> however it is not true if dmem is used, let's teach pat to
> recognize this case if it is ram but it is !pfn_valid()
> 
> We always use WB for dmem and any attempt to change this
> behavior will be rejected and WARN_ON is triggered
> 
> Signed-off-by: Xiao Guangrong <gloryxiao@xxxxxxxxxxx>
> Signed-off-by: Yulei Zhang <yuleixzhang@xxxxxxxxxxx>

Hooks like these will make it very hard to merge this series.

I like the idea of struct page-backed memory, but this is a lot of code
and I wonder if it's worth adding all these complications.

One can already use mem= to remove the "struct page" cost for most of
the host memory, and manage the allocation of the remaining memory in
userspace with /dev/mem.  What is the advantage of doing this in the kernel?

Paolo