Subject: Re: [PATCH V3 01/30] mm: Make vm_get_page_prot arch specific.

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 19 Feb 2016 08:10:00 +0530

Paul Mackerras <[email-masked]> writes:

> On Thu, Feb 18, 2016 at 10:20:25PM +0530, Aneesh Kumar K.V wrote:
>> With next generation power processor, we are having a new mmu model
>> [1] that require us to maintain a different linux page table format.
>> 
>> Inorder to support both current and future ppc64 systems with a single
>> kernel we need to make sure kernel can select between different page
>> table format at runtime. With the new MMU (radix MMU) added, we will
>> have to dynamically switch between different protection map. Hence
>> override vm_get_page_prot instead of using arch_vm_get_page_prot. We
>> also drop arch_vm_get_page_prot since only powerpc used it.
>
> This seems like unnecessary churn to me.  Let's just make hash use the
> same values as radix for things like _PAGE_RW, _PAGE_EXEC etc., and
> then we don't need any of this.
>

I was hoping to do that after this series. Something similar to

https://github.com/kvaneesh/linux/commit/0c2ac1328b678a6e187d1f2644a007204c59a047

"
powerpc/mm: Add helper for page flag access in ioremap_at

Instead of using variables we use static inline which get patched during
boot to either hash or radix version.
"

That gives us a base to revert patches if we find issues with hash and
still have a working radix base. So idea is to introduce radix with minimal
changes to hash and then consolidate hash and radix as much as we can by
updating hash linux format.

-aneesh